FIO-9220: remove hiddenChildren (2.3.x)

[brendanbond]

Link to Jira Ticket

https://formio.atlassian.net/browse/FIO-9220

Description

There is a build/merge problem involving the hideChildren processor. This PR removes the legacy hiddenChildren processor.

Breaking Changes / Backwards Compatibility

n/a

Dependencies

n/a

How has this PR been tested?

n/a

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above