[!!!][TASK] Deprecate unused methods from FormEngine

This patch deprecates the following methods of FormEngine:
* FormEngine::getSingleField_typeFlex_langMenu()
* FormEngine::getSingleField_typeFlex_sheetMenu()
* FormEngine::getSpecConfFromString()
and removes the dependency from all form element classes.

Resolves: #63912
Releases: master
Change-Id: I3c1722785418d384a05028f8ab66b6212a085895
Reviewed-on: http://review.typo3.org/35489
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>