Remove unsused extension check

[NathanReb]

In the list of checks we apply there's one that checks for unused extensions.

Unlike the check for unused attributes it makes no use of the knowledge of declared extensions and simply is run after all transformations, warning about leftover extension points.

Unlike the check for unused attributes it doesn't flag anything that would be silently accepted by the compiler. The compiler already reports uninterpreted extensions.

It seems that besides slowing down the driver when checks are enabled and causing unwanted warnings (see #616), there's no real use for this anymore.

Am I missing something here?

If not I suggest we simply remove entirely.

[NathanReb]

cc @pitag-ha @panglesd if you happen to remember anything about why this check exists, don't hesitate to reach out!