[TASK] Mark various parts with phpdoc internal

All specific controllers for specific Backend actions, Backend-module related modules,
all hook implementations (where the core uses hooks by itself), and module-specific
ViewHelpers are now marked as @internal to ensure developers what is
part of the public TYPO3 Core API.

within
- EXT:felogin
- EXT:feedit
- EXT:filelist
- EXT:frontend
- EXT:fluid_styled_content
- EXT:impexp

All leftover @api methods have been removed.

Resolves: #86508
Releases: master
Change-Id: I97a6964a5f77dc9ee3d90365e9fda3d7d7be6266
Reviewed-on: https://review.typo3.org/58527


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>