[BUGFIX] Update sys_refindex to reflect typolink to file in RTE content

Problem: sys_refindex should be updated when a link to a file is
inserted in or removed from RTE content.
Solution: Add missing softrefs.

Change-Id: Ice1f46a591bd0129aef043b0b18b21be0469157b
Releases: 6.0
Resolves: #36792
Reviewed-on: https://review.typo3.org/10942
Reviewed-by: Steffen Ritter
Reviewed-by: Helmut Hummel
Reviewed-by: Mattias Nilsson
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack