[gdb/testsuite] Remove KFAIL for PR ada/30908

PR ada/30908 turns out to be a duplicate of PR ada/12607, which was fixed by
commit d56fdf1b976 ("Refine Ada overload matching").

Remove the KFAILs for PR ada/30908.

Tested on x86_64-linux.

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

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30908