[TASK] RTE: Use own user agent identification

Replace ExjJs outdated user agent identification.

Releases: master
Resolves: #62630
Change-Id: Ib057fd1ea2c2fc78b0bfdf1e275c69e98331c1c2
Reviewed-on: http://review.typo3.org/33720
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>