gdb: remove empty clear_solib functions

Make the target_so_ops::clear_solib method optional, remove two empty
implementations.

Change-Id: Ifda297d50c74327d337091c58cdb5b3b60382591
Approved-By: Pedro Alves <pedro@palves.net>
Reviewed-By: Reviewed-By: Lancelot Six <lancelot.six@amd.com>
3 files changed