Remove XForms from the manual (part of php/doc-en#3579)

[jimwins]

The XForms content will also need to be removed from each of the doc-$LANG repos.

[jimwins]

Thinking about it further, the correct order to do everything would be to remove all of the references to XForms from all of the translations (references in translated chapters/tutorial.xml and all translated features/xforms.xml) and the reference to features/xforms from chapters/tutorial.xml in php/doc-en, then remove it from manual.xml.in here in php/doc-base, and then finally remove features/xforms.xml from php/doc-en. The entities can finally be removed here. That should leave everything buildable after each PR. (Unless the build fails when there are unused entities?)

[Girgias]

Unused entities are fine.

You will need to do the doc-en changes first, as the translators need to update the EN-Revision tag, I have pushed major changes by using a revcheck skip but I try to limit my usage of those.

[jimwins]

Going to go ahead and merge this now that it's just three languages (it, pl, tr) that haven't pushed the submitted PR to remove this. They can clean up after the fact.