emacs: Reuse rather than reinvent message header filtering

emacs: Reuse rather than reinvent message header filtering

In notmuch-mua-reply we were filtering out the Subject and To headers
manually in a loop, but message mode offers a nice function for
exactly that. Simplify the code by using it. Also, as notmuch-mua-mail
already sorts and hides headers that we want sorted and hidden, we can
safely remove those 2 functions from here as well.  Also remove the
(require 'cl), the only reason for its existence was the now removed
"loop" function.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>