CXX-151 As part of RC3 backport, drop signal_handlers.

These files changed significantly between rc2 and rc3, including
pulling in a new dependency on db/client.h, which we had not extracted.

We def. don't want db/client.h and its transitive dependencies, but
as a nice consequence of other edits in rc2..rc3, no files that we
own now depend on signal_handlers.h, so we can drop the dependency,
rather than needing to pull in db/client.h

Note that we have already made this change on the legacy branch. The
driver library shouldn't be in the business of signal handling.

src/SConscript.client

@@ -98,7 +98,6 @@ clientSourceBasic = [
     'mongo/util/net/sock.cpp',
     "mongo/util/net/socket_poll.cpp",
     'mongo/util/net/ssl_manager.cpp',
-    'mongo/util/signal_handlers.cpp',
     'mongo/util/stringutils.cpp',
     'mongo/util/text.cpp',
     'mongo/util/time_support.cpp',
@@ -231,7 +230,6 @@ clientHeaders = [
     'mongo/util/net/sock.h',
     'mongo/util/net/ssl_manager.h',
     'mongo/util/net/ssl_options.h',
-    'mongo/util/signal_handlers.h',
     'mongo/util/time_support.h',
     'mongo/util/version.h',
 ]