Revert "[FEATURE] EXT:form - introduce YAML "imports""

While the intended feature is great (having the possiblity
to extract and re-use parts of the form definition),
implementation wise it needs some rework.

To be able to release 9.1 as stable as possible,
we now revert this implementation for now
and bring it in for 9.2 again.

This reverts commit a52a9922.

Reverts: #82089
Resolves: #83729
Releases: master
Change-Id: I850401d232b642e712801613d3780816f873966a
Reviewed-on: https://review.typo3.org/55422


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>