Eliminate no-op MandatoryLeafEnforcer

Eliminate no-op MandatoryLeafEnforcer

Running through a no-op enforcer, while efficient, still requires
a bimorphic invocation, which results most of the time in a no-op.

Eliminate the no-op implementation and refactor its two users, so
that they provide dedicated subclasses which use enforcer. This has
the benefit of not overriding SchemaAwareApplyOperation just to
run a noop invocation.

JIRA: YANGTOOLS-945
Change-Id: Iff44eb6efa048bb86a71b0d7753d3e58425b5e4d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>