Remove email sending functionality (google#6)

This removes email sending functionality as per yugabyte/yugabyte-db#11930.

The relevant gflags remain in place to avoid any issues with clusters that already have the flags set, but no longer do anything.
# Conflicts:
#	src/logging.cc

# Conflicts:
#	src/logging.cc

src/logging.cc

@@ -2320,98 +2320,12 @@ static inline void trim(std::string &s) {
 // log_mutex.
 static bool SendEmailInternal(const char*dest, const char *subject,
                               const char*body, bool use_logging) {
-#ifndef GLOG_OS_EMSCRIPTEN
-  if (dest && *dest) {
-    // Split the comma-separated list of email addresses, validate each one and
-    // build a sanitized new comma-separated string without whitespace.
-    std::istringstream ss(dest);
-    std::ostringstream sanitized_dests;
-    std::string s;
-    while (std::getline(ss, s, ',')) {
-      trim(s);
-      if (s.empty()) {
-        continue;
-      }
-      // We validate the provided email addresses using the same regular
-      // expression that HTML5 uses[1], except that we require the address to
-      // start with an alpha-numeric character. This is because we don't want to
-      // allow email addresses that start with a special character, such as a
-      // pipe or dash, which could be misunderstood as a command-line flag by
-      // certain versions of `mail` that are vulnerable to command injection.[2]
-      // [1] https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address
-      // [2] e.g. https://nvd.nist.gov/vuln/detail/CVE-2004-2771
-      if (!std::regex_match(
-              s,
-              std::regex("^[a-zA-Z0-9]"
-                         "[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]*@[a-zA-Z0-9]"
-                         "(?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\\.[a-zA-Z0-9]"
-                         "(?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"))) {
-        if (use_logging) {
-          VLOG(1) << "Invalid destination email address:" << s;
-        } else {
-          fprintf(stderr, "Invalid destination email address: %s\n",
-                  s.c_str());
-        }
-        return false;
-      }
-      if (!sanitized_dests.str().empty()) {
-        sanitized_dests << ",";
-      }
-      sanitized_dests << s;
-    }
-    dest = sanitized_dests.str().c_str();
-
-    if ( use_logging ) {
-      VLOG(1) << "Trying to send TITLE:" << subject
-              << " BODY:" << body << " to " << dest;
-    } else {
-      fprintf(stderr, "Trying to send TITLE: %s BODY: %s to %s\n",
-              subject, body, dest);
-    }
-
-    string logmailer = FLAGS_logmailer;
-    if (logmailer.empty()) {
-        logmailer = "/bin/mail";
-    }
-    string cmd =
-        logmailer + " -s" +
-        ShellEscape(subject) + " " + ShellEscape(dest);
-    if (use_logging) {
-        VLOG(4) << "Mailing command: " << cmd;
-    }
-
-    FILE* pipe = popen(cmd.c_str(), "w");
-    if (pipe != nullptr) {
-      // Add the body if we have one
-      if (body) {
-        fwrite(body, sizeof(char), strlen(body), pipe);
-      }
-      bool ok = pclose(pipe) != -1;
-      if ( !ok ) {
-        if ( use_logging ) {
-          LOG(ERROR) << "Problems sending mail to " << dest << ": "
-                     << StrError(errno);
-        } else {
-          fprintf(stderr, "Problems sending mail to %s: %s\n",
-                  dest, StrError(errno).c_str());
-        }
-      }
-      return ok;
-    } else {
-      if ( use_logging ) {
-        LOG(ERROR) << "Unable to send mail to " << dest;
-      } else {
-        fprintf(stderr, "Unable to send mail to %s\n", dest);
-      }
-    }
+  const char* error = "Sending emails is disabled (#11930).";
+  if ( use_logging ) {
+    LOG(ERROR) << error;
+  } else {
+    fputs(error, stderr);
   }
-#else
-  (void)dest;
-  (void)subject;
-  (void)body;
-  (void)use_logging;
-  LOG(WARNING) << "Email support not available; not sending message";
-#endif
   return false;
 }