pgsql: Remove support for libmhash/libmcrypt.

Log Message:
-----------
Remove support for libmhash/libmcrypt.

libmcrypt seems to dead, maintainer address bounces,
and cast-128 fails on 2 of the 3 test vectors from RFC2144.

So I see no reason to keep around stuff I don't trust
anymore.

Support for several crypto libraries is probably only
confusing to users, although it was good for initial
developing - it helped to find hidden assumptions and
forced me to create regression tests for all functionality.

Marko Kreen

Modified Files:
--------------
    pgsql/contrib/pgcrypto:
        Makefile (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/Makefile.diff?r1=1.12&r2=1.13)
        README.pgcrypto (r1.7 -> r1.8)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/README.pgcrypto.diff?r1=1.7&r2=1.8)

Removed Files:
-------------
    pgsql/contrib/pgcrypto:
        mhash.c
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pgcrypto/mhash.c)