Remove mentions of libbacktrace from COPYRIGHT

Use of libbacktrace was removed in
06d565c where we switched to using the
gimili library instead. Note: the backtrace submodule located at
library/backtrace points to backtrace-rs which removed support for using
libbacktrace in rust-lang/backtrace-rs#423.

COPYRIGHT

@@ -110,41 +110,3 @@ their own copyright notices and license terms:
     NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
     USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
     OF SUCH DAMAGE.
-
-* libbacktrace, under src/libbacktrace:
-
-    Copyright (C) 2012-2014 Free Software Foundation, Inc.
-    Written by Ian Lance Taylor, Google.
-
-    Redistribution and use in source and binary forms, with
-    or without modification, are permitted provided that the
-    following conditions are met:
-
-        (1) Redistributions of source code must retain the
-        above copyright notice, this list of conditions and
-        the following disclaimer.
-
-        (2) Redistributions in binary form must reproduce
-        the above copyright notice, this list of conditions
-        and the following disclaimer in the documentation
-        and/or other materials provided with the
-        distribution.
-
-        (3) The name of the author may not be used to
-        endorse or promote products derived from this
-        software without specific prior written permission.
-
-    THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND
-    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
-    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
-    AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN
-    NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
-    INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-    CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
-    PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
-    USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
-    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-    CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
-    NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
-    USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
-    OF SUCH DAMAGE.  */