[BUGFIX] Clean up caching framework garbage collection task

Clean up some obsolete code from the garbage collection task since the
caching framework was enabled by default. Additionally fixes the
additional fields provider to find registered cache backends.

Change-Id: I242a9ccb904f9e692025336a6e302df673078050
Resolves: #28062
Reviewed-on: http://review.typo3.org/4427
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog