Special mailboxes obviously can't be located on remote accounts, so don't show them in preferences

Bug #13648
[mms] Remove the 'display_folder' hook.
phpdoc
[mms] Remove 'mailboxarray' hook.

This is not necessary anymore, now that template code can directly execute
PHP. And the template code needs to be modified anyway to handle the
array additions.
Fix saveToSentMail call

Signed-off-by: Michael M Slusarz <slusarz@horde.org>