cmd/gethrpctest: ethereum/rpc-tests is deprecated

Only ethereum/rpc-tests used this command, which hasn't been maintained for over
a year now, a lot of tests failing. What's left of it was moved underneath hive,
which can run the entire test against a black-box geth without special commands.

Also a new RPC test suite is being added which is also based on black box tests,
not needing special commands any more.