[Discussion]Remove custom rpc methods except mantapay/mantasbt's

[Dengjianping]

Currently all rpc methods can be queried from runtime APIs by state_call. Just like

api.rpc.state.call('method_name', bytes);

Pros:

• No need to maintain custom rpc crates.
• Node redeployment will become unnecessary(it depends).
• Shorten compile time(maybe a little bit), and shrink the size of client(maybe a little bit as well).

Cons:

• Performance. But if no extra decoding/encoding in rpc methods, performance should be affected.
• Frontend. Frontend team has to rewrite the part of calling custom rpc methods.