Disable GNU rcs by default

All remaining tools using rcs has been switched to directly use diff3(1):

• etcupdate(8)
• freebsd-update(8)

Note that the ident(1) tool is been already replaced long ago with a BSD
licensed version, as such it remains installed.

GNU rcs is still available from ports:

• rcs: newer GPLv3 version
• rcs57: the latest version from base (GPLv2)