[BUGFIX] Avoid reference index records for images in RTE fields

Since images in RTE fields are handled as relations to sys_file
records adding references to the reference index for the concrete
image paths are obsolete and causes trouble with impexp.

Resolves: #56811
Releases: 6.2
Change-Id: I26a2531f417ff8b628b69156a3a2236c99960d5e
Reviewed-on: https://review.typo3.org/28292
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs