Currently because the Get mixin is not generic every subclass has
to implement its own `get` methods to type hint `get` method return
beoynd the basic RESTObject.

The upcoming PR will change Get mixin to use generics. This means
it will be able to type hint the corresponding `_obj_cls` and no
`get` method would need to be redefined.

Because removing existing `get` methods modifies a lot of files
split it in to a separate commit.

Also remove the `tests/unit/meta/test_ensure_type_hints.py` file
because testing subclasses for `get` methods is no longer relevant.

Signed-off-by: Igor Ponomarev <igor.ponomarev@collabora.com>