[TASK] Remove empty .gitmodules file

With removal of git submodules the .gitmodules file is empty
and can be removed now.

Change-Id: I453f63c0852825b46e9ea62f87385d9eace25134
Resolves: #50480
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/22613
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn