[TASK] Remove FrontendFormEngine class

Last core usage of FrontendFormEngine was removed in 6.2 with a
refactoring of feedit. The class was only a slight extension of
FormEngine. Since the FormEngine works is breaking and also
documented as such, the class is now dropped without further
deprecation - it is also relatively unlikely any currently
maintained still relies on it.

Resolves: #67979
Releases: master
Change-Id: I766dade040725b468572e6f303786af035bc2f2b
Reviewed-on: http://review.typo3.org/40998


Reviewed-by: Thomas Schlumberger <thomas@b13.de>
Tested-by: Thomas Schlumberger <thomas@b13.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>