[gdb/tui] Assume HAVE_WBORDER

The tui border-kind setting allows values acs, ascii and space.

The values ascii and space however don't work well with !HAVE_WBORDER.

Fix this by removing the !HAVE_WBORDER case, which was introduced for Ultrix
support, which is now obsolete.

Tested on x86_64-linux.

PR tui/30580
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30580

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