The test for `gettxoutsetinfo` in `rpc_blockchain.py` verifies that the result is the same as before after invalidating and reconsidering a block.  The comparison has to exclude the `disk_size` field, though, as it is not deterministic.

Instead of comparing all the other fields for equality, this change explicitly removes the `disk_size` field and then compares the full objects.  This makes the intent more explicit (compare everything except for `disk_size`, not compare just a given list of fields) and also the code simpler.

./test/functional/test_runner.py rpc_blockchain