notmuch-emacs-mua: remove -C to keep short options compatible with mutt

Notmuch uses long options exclusively all around. The short options in
notmuch-emacs-mua are intentionally just a compatible subset of
mutt(1). Keep it this way, if only to make documenting the fact easy!

The Notmuch style --client long option remains, of course.

doc/man1/notmuch-emacs-mua.rst

@@ -18,7 +18,7 @@ Supported options for **notmuch-emacs-mua** include
     ``-h, --help``
         Display help.
 
-    ``-C, --client``
+    ``--client``
         Use emacsclient, rather than emacs. This will start
         an emacs daemon process if necessary.
 

notmuch-emacs-mua

@@ -38,7 +38,8 @@ CLIENT_TYPE="-c"
 ELISP="(prog1 'done (require 'notmuch) (notmuch-mua-new-mail)"
 ELISP="${ELISP} (setq message-exit-actions (list #'save-buffers-kill-terminal))"
 
-while getopts :s:c:b:i:hC opt; do
+# Short options compatible with mutt(1).
+while getopts :s:c:b:i:h opt; do
     # Handle errors and long options.
     case "${opt}" in
 	:)
@@ -79,7 +80,7 @@ while getopts :s:c:b:i:hC opt; do
 	--help|h)
 	    exec man notmuch-emacs-mua
 	    ;;
-	--client|C)
+	--client)
 	    USE_EMACSCLIENT="yes"
 	    ;;
 	--subject|s)