Revert "git-am: catch missing author date early."

Revert "git-am: catch missing author date early."

This reverts commit 6e9e0327b7d7f384d8a223b4bc40330ef3e7fb61.  People
can prepare a text file with Subject: and From: headers and feed it to
"am" (pretending the file is a piece of e-mail), and have actually been
doing so.  Strict checking for Date: breaks this established workflow,
which wants to record the time of the commit as the author time.

Thanks go to Jens Axboe for injection of sanity.

Signed-off-by: Junio C Hamano <gitster@pobox.com>