[BUGFIX] Fix broken RTE within flexform

In my case with EXT:flux and TYPO3 7.6 the RTE doesn't appear. This is caused by a syntax error within the JavaScript. The selector of the content element text field looks actually like $('.tinymce4_rte#RTEarea'data_tt_content__55__bodytext_''), where are single quotes arround the "editornumber". This is caused by addConfigurationOption(), where strings get wrapped.

This fix removes the single quotes around the string and respects the previous fix in Commit e699df35.