gdb: Remove stabs support from ELF files

This commit makes it so that GDB won't read stabs information from ELF
files.  If stabs is detected in an ELF file, the reader now warns the user
that stabs is not supported.

This test would cause two new failures in the test gdb.stabs/weird.exp
(that surprisingly didn't happen in the mips commit).  Rather than fixing
the test that'll be removed soon, I just removed the test instead.

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