[TASK] Remove superfluous timestamp CKEditor plugin

With #96874, the timestamp example plugin for CKEditor has been
added to `ext:rte_ckeditor`. The plugin is however not used or
referenced anywhere, so it can safely be removed.

Resolves: #100781
Releases: main
Signed-off-by: Torben Hansen <derhansen@gmail.com>
Change-Id: I2fe97a260f5ca0703684fbc324fad172eedf6d08
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78912


Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>