Revert "lib: tdb: Use sigaction when testing for robust mutexes."

Revert "lib: tdb: Use sigaction when testing for robust mutexes."

This fails on Linux platforms with robust mutex support with the
following error:

tdb(/home/asn/workspace/projects/samba/git/st/nt4_dc/lockdir/gencache_notrans.tdb):
    tdb_mutex_open_ok[/home/asn/workspace/projects/samba/git/st/nt4_dc/lockdir/gencache_notrans.tdb]:
    Can use mutexes only with MUTEX_LOCKING or NOLOCK

We also see winbind is not able to start with this error message trying
to open the serverid.tdb.

This reverts commit d1914367289b58f26544ee6e116490d662d9c41c.

Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Wed Mar 25 14:58:38 CET 2015 on sn-devel-104