devel/valgrind: Update to 3.18.1 and switch to use sourceware

The FreeBSD port of Valgrind has now been merged upstream and a new
release of Valgrind is out, 3.18.1. This updates Makefile, distinfo and
pkg-descr to reflect that.

Also remove pkg-message which has been incorrect and obsolete for a fair time.

PR: 259224