Remove USE_FIREBIRD from bsd.databases.mk, replaced by USES=firebird.

Also:

• replace USE_FIREBIRD by USES=firebird.
• convert ports depending on databases/firebird25-client directly to USES=firebird.

PR: 208971
Submitted by: mat
Exp-run by: antoine
With hat: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D5951