Adjust link browser integration of RTE.

This removes the dedicated tree classes for RTE as those are useless
by now. Additionally the ElementBrowser class is finally removed.

Resolves: #66369
Releases: master
Change-Id: I8f42228cb3c4b3008d3f0acaa014d01733f6186d
Reviewed-on: https://review.typo3.org/43938


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>