[!!!][TASK] Remove deprecated EXT:form constants and functions

This change removes following public class constants:

* EmailFinisher::FORMAT_PLAINTEXT
* EmailFinisher::FORMAT_HTML

Following internal functions are removed:

* FormEditorController::migrateTranslationFileOptions()
* FormEditorController::migrateEmailFormatOption()

There were also some deprecated internal functions which
got already removed / changed in #90728.

Resolves: #92389
Releases: master
Change-Id: I241f363d03d9e58bb5e4788a7c1c7262f962f585
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65842


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>