[BUGFIX] Remove isLoaded('fluid') call from FLUIDTEMPLATE

EXT:fluid is a required extension since TYPO3 6.0. The
isLoaded('fluid') call in FLUIDTEMPLATE content object
can be removed.

Change-Id: I514163bffc19fa264501dc2d88e1fe59f0e9a062
Fixes: #43172
Releases: 6.0
Reviewed-on: http://review.typo3.org/16633
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn