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

This reverts commit 6e9e0327b7.  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>

git-am.sh

@@ -307,9 +307,9 @@ do
 	GIT_AUTHOR_EMAIL="$(sed -n '/^Email/ s/Email: //p' "$dotest/info")"
 	GIT_AUTHOR_DATE="$(sed -n '/^Date/ s/Date: //p' "$dotest/info")"
 
-	if test -z "$GIT_AUTHOR_EMAIL" || test -z "$GIT_AUTHOR_DATE"
+	if test -z "$GIT_AUTHOR_EMAIL"
 	then
-		echo "Patch does not have valid authorship information."
+		echo "Patch does not have a valid e-mail address."
 		stop_here $this
 	fi