src.opts.mk: Remove REPRODUCIBLE_BUILD from the default list

It was added to the list in bsd.opts.mk in commit 4f81c42fbd76, so
should have been removed here.

Reported by: cperciva
Fixes: 4f81c42fbd76 ("share/mk: Substitute reproducible prefixes in dwarf info")
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.