Revert "[FEATURE] Add filemount property to backend users"

This reverts commit c7313d09d0cb2335d628cb55d73372f2c41447c4

File mounts a graph in CMS core. This is not reflected
by the current implementation. The feature is reverted to not add
a half baked solution.

Change-Id: Ibb2ff41db10f3daa0842e1acf52058489b81045e
Releases: 6.2, 6.1
Related: #39324
Reviewed-on: https://review.typo3.org/20303
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn