Remove mentions of libbacktrace from COPYRIGHT

Use of libbacktrace was removed in
06d565c967bfb7c6ff52a991bbe47b4a2a25de3e 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 https://github.com/rust-lang/backtrace-rs/pull/423.