yang.data.impl.schema.tree clean-up

yang.data.impl.schema.tree clean-up

* Propagate @Nonnull and @Nullable declarations.
* Suppress unused warnings for AbstractCursor's closed field.
* Remove redundant generic type declarations.
* In AbstractDataTreeCandidateNode, make sure that either newData or
  oldData is not null (or both).
* Remove CaseEnforcer::enforceOnTreeNode() which is unused.
* Remove unnecessary semi-colons in MandatoryLeafEnforcer.
* Remove MinMaxElementsValidation::checkOriginalPresent() which is
  unused.
* SchemaAwareApplyOperation::checkConflicting indicates its result
  using an exception, so drop the return value which is always true
  (this method isn't used anywhere else in Boron).
* SchemaAwareApplyOperation::checkNotConflicting is static, so its
  final modifier is redundant.

Change-Id: I7f1ebe53753edd0b0ec5c333ac0cc2850c959f15
Signed-off-by: Stephen Kitt <skitt@redhat.com>