Remove the restriction from RecursiveContextualValidator that prevents using custom groups.

Problem/Motivation

While working in #3441434: Add validation constraints to core.menu.schema.yml we found that the RecursiveContextualValidator prevents us from using custom groups. But we want to use AtLeastOneOf constraint there and it's validator requires every constraint to have a group.

Steps to reproduce

Proposed resolution

Remove the exception from RecursiveContextualValidator that prevents us from using custom groups.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet