Remove {Numerical,String}RestrictionsImpl

Remove {Numerical,String}RestrictionsImpl

As noted during previous migration, these classesalong with their
support is unused. Furthermore NumericalRestrictions is
mis-declared.

Remove the implementation and drop a FIXME for next major version,
so that we at least fix the definition or decide its overall
future.

Note this concept is probably useful, as it gates towards tasks
which are currently done by AbstractTypeStatementSupport and those
could be split out if we rework interactions with simple built-in
types.

JIRA: YANGTOOLS-1065
Change-Id: Ief43d59dbc541b094ca80e39382f8c0c8767bac2
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>