[TASK] RTE: Avoid use of Ext.isString and Ext.isFunction

Avoid useless overhead.

Releases: master
Resolves:#62617
Change-Id: Ibb722b405d8c95335f655c78faa05fe3be8a4ea6
Reviewed-on: http://review.typo3.org/33706
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>