Drop obsolete keystore_socket type and rules.

Change I6dacdc43bcc1a56e47655e37e825ee6a205eb56b switched
the keystore to using binder instead of a socket, so this
socket type and rules have been unused for a while.  The type
was only ever assigned to a /dev/socket socket file (tmpfs) so
there is no issue with removing the type (no persistent files
will have this xattr value).

Change-Id: Id584233c58f6276774c3432ea76878aca28d6280
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>