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.

Sponsored by: The FreeBSD Foundation
Sponsored by: Klara, Inc.

(cherry picked from commit b2f5dc591ef2547ee2e9b68e58a263cd34948db1)
(cherry picked from commit 7626ba028089b97b5bb204b5203ee8fa24f63c48)

PR: 290427