Removed signing_dir from neutron.conf

The default value from keystone middleware ($TMPDIR) is good enough,
so stick to it.

Closes-Bug: 1313780
Change-Id: Ideb393f9010f4f1706392ccdfdd5fa7307bda070
(cherry picked from commit e423126965fb79d35f5f2281974cee6a216af8d4)