[TASK] RTE: Remove unused template file

File templates/rtehtmlarea_pageheader_frontend.html is not used since
this change: https://review.typo3.org/#/c/15413/

Releases: master
Resolves: #62806
Change-Id: I57e23b15638098632b49b0235b725b7cd8e48dea
Reviewed-on: http://review.typo3.org/33937
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>