[CLEANUP] Remove references to $GLOBALS['FILEMOUNTS']

Global filemounts are not supported with FAL anymore
as the implementation is done via file storages directly.
All references to the global FILEMOUNTS is removed as
it was an empty array since 6.0.

Next step (but not part of this patch) is to clean up the
functions in BasicFileUtility that make use of the filemounts.

Releases: 6.2
Resolves: #55547
Change-Id: I07e08afe54c174d1c3a67c41645819e5831fd1e3
Reviewed-on: https://review.typo3.org/27227
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters