[CLEANUP] Drop the legacy manual.sxw file

The old manual has long been replaced by the ReST documentation.

Resolves: #63635

Change-Id: I8e1f64dbb1578d26c2b76ec6029268cf913cc3d6
Reviewed-on: http://review.typo3.org/35115
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>