[gdb/testsuite] Remove gdb.base/gdbindex-stabs.exp

On openSUSE Leap 15.6 x86_64 I ran into:
....
(gdb) file gdbindex-stabs^M
Reading symbols from gdbindex-stabs...^M
warning: stabs debug information is not supported.^M
(gdb) list stabs_function^M
(gdb) FAIL: gdb.base/gdbindex-stabs.exp: list stabs_function
...

Fix this by removing the test-case.

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