[CLEANUP] Remove obsolete isLoaded(workspaces) in ext:workspaces

A check for isLoaded('workspaces') in ext:workspaces is obsolete
since the class can only be used if the extension is loaded. Remove
the check.

Resolves: #62842
Releases: master
Change-Id: I1aa305921e57196df6d317b068080e96d137c4a0
Reviewed-on: http://review.typo3.org/33972
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>