gdb: remove unnecessary nullptr check in remove_user_added_objfile

Since commit 74daa597 ("gdb: add all_objfiles_removed observer"), the
objfile passed to the free_objfile observable can't be nullptr.

Change-Id: If215aa051ab43c068b11746938022c7efca09caa
Approved-By: Andrew Burgess <aburgess@redhat.com>