[BUGFIX] Remove EXT:cli from REQUIRED_EXTENSIONS

EXT:cli was removed and merged with core but the extension key is still in
the list of REQUIRED_EXTENSIONS.

Change-Id: I272338b2676f8d84d586586c36c151c9339ed1a7
Resolves: #42138
Related: #40949
Releases: 6.0
Reviewed-on: http://review.typo3.org/15788
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn