devel/gdb: Remove the EXPAT option.

GDB on FreeBSD requires libxml for full funtionality (e.g. 'catch
syscall <syscall name>' parses the XML files installed in
LOCALBASE/share that were still installed even when this option was
disabled). Also, using GDB with a debug server that returns any
non-simple registers requires XML support for parsing the target
description returned by the server (e.g. examining SSE or later
registers or any system registers when attached to QEMU or
lldb-server).

Reviewed by: pizzamig (maintainer), danfe
Differential Revision: https://reviews.freebsd.org/D38760