Remove XForms documentation (part of php/doc-en#3579)

[jimwins]

Removes the translation in advance of it being removed entirely.

[cmb69]

Thank you! Before this can be merged, php/doc-en#3601 needs to be merged, and then the EN-Revision for the modified file needs to be updated, and possibly other pending updates to that file need to be made.

[jimwins]

Updated the revision. The reason for going ahead and deleting features/xforms.xml now, in this and other translations, is so that when reference to it removed from doc-base/manual.xml.in it won't immediately break the build for all of those translations. In the (hopefully short) time between it being removed from the translations and from doc-en, the translations should just fall back to the English version.

[jimwins]

Looked in the wrong branch and thought this had been done. Still needs doing.

[cmb69]

Maybe we should not update the EN-Revision here; there may have been other changes to this file in between (or haven't there any). Not sure what's best for the active translators; maybe @msamesch can comment?

[jimwins]

There were no other untranslated changes to that file, so chopping out that paragraph and updating the EN-Revision should bring it up to date. (I made PRs for every language, but only changed the EN-Revision if there were no other untranslated changes, was just doc-es and doc-it. The change without updating EN-Revision was merged to doc-es and doc-it is still pending for someone to deal with.)

[msamesch]

I forgot about this PR while I was waiting for the corresponding update in EN. Otherwise it's fine with me. Also meanwhile chapters/tutorial.xml was updated in c73ea82. Thanks for the update anyway.

[cmb69]

@jimwins, as far as I know, doc-it is broken for weeks (if not longer), so probably no need to worry about another break.