[TASK] Use ajax.php for clickmenu

The clickmenu still used its own alt_clickmenu.php
entry point until now which is not needed anymore
with ajax.php.

Further steps (out of scope for this patch)
are: use JSON instead of proprietary XML
and jQuery instead of prototype.

Releases: master
Resolves: #62666
Change-Id: Iae4a9623c59a8c985d15dbc9be0c0544ce6abf8e
Reviewed-on: http://review.typo3.org/33763
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>