[BUGFIX] Fatal error: "enableFields on non-object" in extension manager

Don't rely on $TSFE object to detect if we are in a frontend context. Rely
on TYPO3_MODE constant instead.

Releases: 6.2, 6.1, 6.0
Resolves: #50803
Change-Id: I06ea2692dab7683c7284e5f3ff45036d72c64999
Reviewed-on: https://review.typo3.org/22830
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind