gdb: remove unnecessary tui directory check in configure

I suppose this was possible in the CVS days for the tui directory to be
missing, but it's not really possible nowaday.  Well, a user could
delete the directory from their source tree but... it doesn't make
sense.  Remove the check for that directory in configure.

Change-Id: Iea1412f5e5482ed003015030132ec22150c7d0b3
Approved-By: Tom Tromey <tom@tromey.com>