build: Use rm -fv for BATCH_DELETE_OLD_FILES

It's possible to have files with odd permissions in the tmproot (or
sysroot), causing rm to prompt for each one during e.g. buildworld.
Add -f to forcibly delete these.

Reviewed by: brooks
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37111