[TASK] Remove unused internal methods from ExtensionManagementUtility

The internal methods `generateItemList()` and `explodeItemList()`
from `ExtensionManagementUtility` are unused since v7 and therefore
now removed.

Resolves: #94314
Releases: master
Change-Id: I6db5cf6e198f7d69ecb58c5ca8dec1e24c9c55a4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69463


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>