gdb: remove redundant rm in local-maintainer-clean

The local-maintainer-clean target removes a hardcoded list of
parser-generator output files, and then removes $(YYFILES) two lines
below.  $(YYFILES) contains the same list of files, so the first rm is
redundant.  Remove it.

Change-Id: I14d27930953e33d54010d964cb4bb7db4e8323a0
Approved-By: Kevin Buettner <kevinb@redhat.com>