gdb: Fully remove stabs code from GDB

This commit is the last in the series removing GDB's support for stabs.
It removes the stabsread.{c|h} files, and removes the last usage of
stabsread stuff in buildsym.  Notably, the header file gdb-stabs.h
remains in the tree as it is misnamed at this point - it is used for
reading dbx objfiles.  It (and dbxread) will be removed in a future
commit.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>