Simplify comparison in rpc_blockchain.py.
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