[!!!][TASK] Remove option to allow uploading system extensions

The extension manager has the option to upload an extension
into typo3/sysext/, which is no recommendation and should
not be an option from the Backend UI itself, as it is heavily
conflicting with core updates where typo3_src/ / sysext is
exchanged.

The option is removed without substitution, if somebody
wants to play with typo3/sysext/ it's at the admins own risk,
and should not be encouraged or mentioned neither in the install
tool (All Configuration) nor Extension Manager.

Resolves: #82377
Releases: master
Change-Id: I19782a4960bb52bd2012b79e912e483610d297d8
Reviewed-on: https://review.typo3.org/54008


Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>