[TASK] Housekeeping in Extension Manager Code

This patch mainly:
- Does use statements in all EM code
- Removes unused fields "sponsorlink", "sponsorlogo" and "sponsorname" of mirrors XML
- Removes unused methods in internal classes

It is time to clean up... First one is to get an overview of what is still needed.

Resolves: #89751
Releases: master
Change-Id: Ie30f05f413afc259fdef6064c363fff47483b331
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62391


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Susanne Moog <look@susi.dev>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>