Revert "[TASK] Configurable width of the Element Browser"

Reverted per request of the RM because:
- there is a merge freeze for regressions
- 6.2 is now a stable branch (no new features)

Besides:
- no documentation issue
- not enough votes

This reverts commit 8f520011793f558a5dad33f6a51ecabc5cef41c1

Change-Id: Ia1ffee069f825b678748808d756a2c04fd8edb18
Reviewed-on: https://review.typo3.org/29616
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert