git-send-email: do not pass custom Date: header

git-send-email: do not pass custom Date: header

We already generate a Date: header based on when the patch was
emailed.  git-format-patch includes the Date: header of the
patch.  Having two Date: headers is just confusing, so we
just use the current Date:

Often the mailed patches in a patch series are created over a
series of several hours or days, so the Date: header from the
original commit is incorrect for email, and often far off enough
for spam filters to complain.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>