This behavior was inconsistent with other editor-only nodes such as
Position3D, Position2D and ReferenceRect. It also caused issues when
a script extended RootMotionView as it ceased to work when the project
was run.
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
