Old STABS code is still being compiled, but never used. Remove it.

Since valgrind 3.9.0 the STABS support was already disabled completely.
But the code was still there being compiled and we were still searching
for stabs sections in binaries. Completely remove all sources, tests and
references. Add a note to coregrind/m_debuginfo/README.txt to mention
the old code can be found in the subversion repository.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14550 a5019735-40e9-0310-863c-91ae7b9d1cf9