databases/Makefile: Remove hook of deleted port

It appears that the port databases/mariadb106-client has been removed,
but an entry for it still exists in databases/Makefile.

PR: 296426
Reported by: M. Voorhis <mvoorhis@mcvau.net>
Approved by: osa, vvd (Mentors, implicit)
Fixes: 0df4ffbdef34 ("databases/mariadb106-client: remove epired port")