Convert createlang/droplang to use CREATE/DROP EXTENSION.

Convert createlang/droplang to use CREATE/DROP EXTENSION.

In createlang this is a one-line change.  In droplang there's a whole
lot of cruft that can be discarded since the extension mechanism now
manages removal of the language's support functions.

Also, add deprecation notices to these two programs' reference pages,
since per discussion we may toss them overboard altogether in a release
or two.