small cleanup for s_lock.h

I noticed that s_lock.h points to a default implementation of tas() in
tas.s or s_lock.c, but AFAICT there hasn't been a tas() implementation in
s_lock.c since commit 718aa43a4e, and commit 25f36066dd seems to have
removed the last remaining tas.s files.  So, I think this is dead code.

I also noticed that HAS_TEST_AND_SET just means that TAS is defined, so I
wrote a 0002 that removes it in favor of checking TAS directly.  I'd like
to rewrite the comment at the top of the file, too, but haven't gotten to
that yet.  I find it a little misleading, especially because we #error if
TAS isn't defined.

-- 
nathan