Revert "[Form] Fix of "PATCH'ed forms are never valid""

This reverts commit a2b15359d81563d52b62b4357407efa949a0706a.

Conflicts:
	src/Symfony/Component/Form/Form.php

The commit is reverted because it introduces a bug demonstrated by a currently failing test.
[Form] Fixed patched forms to be valid even if children are not submitted
merged branch bschussek/issue8111 (PR #8638)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Fixed patched forms to be valid even if children are not submitted

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8111
| License       | MIT
| Doc PR        | -

#8362 was reverted because it introduces a failing test that is caused by a regression. This PR takes the alternative approach that

* unsubmitted fields in the PATCH request remain unsubmitted
* `isValid()` ignores unsubmitted children
* `mapFormsToData()` ignores unsubmitted children

In my opinion this is a more proper solution than #8362.

Commits
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85330a6 [Form] Fixed patched forms to be valid even if children are not submitted
50f201e Revert "[Form] Fix of "PATCH'ed forms are never valid""