Remove SchemaNode hashCode()/equals()

Our SchemaNode implementations have pretty much arbitrary hashCode/equals implementations, mostly centered around SchemaPath. These are not documented anywhere in the interface specification.

Specifying these is rather troublesome because of the number of classes involved and the requirement of equals() being reflection. This leads to equality not being useful beyond SchemaContext-specific caches, where defaults from java.lang.Object perform better.

Audit all SchemaNode implementations and remove overridden hashCode/equals methods.

If a requirement to compare SchemaNodes surfaces, such as cross-SchemaContext data migration, that functionality needs to be exposed as a separate API.