ART: Clean up checks in CheckProxyMethod().

The non-proxy method, `np`, is always the same as the
`prototype`, so it's not really worth comparing their
members. That's been the case since
    https://android-review.googlesource.com/148090 .

We've see the comparison of the return type hit a CHECK()
failure for ObjPtr<> cookie. This can happen since
    https://android-review.googlesource.com/431679
if the return type is evicted from the DexCache and we go
through the slow path.

Test: m test-art-host-gtest
Test: testrunner.py --host
Bug: 30627598
Change-Id: I11f9ea86326be6ef9043ac5f170d74165ac11343