replay: remove HEAD related sanity check

replay: remove HEAD related sanity check

We want replay to be a command that can be used on the server side on
any branch, not just the current one, so we are going to stop updating
HEAD in a future commit.

A "sanity check" that makes sure we are replaying the current branch
doesn't make sense anymore. Let's remove it.

Co-authored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>