[DOCS] Remove unused include file from EXT:form docs

With #89129 the containerClassAttribute.rst include for Fieldset
was removed but the file which is now unused was not removed.

Reaolves: #89214
Related: #89129
Releases: master
Change-Id: Ib0055bcf3f8f9d256b50c167b37d454ebed145b2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61769
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>