Remove PoW related APIs

[smartprogrammer93]

In addition to #328. I am thinking it makes sense to remove PoW related APIs from the spec at this point. This includes the following methods:

1. eth_mining
2. eth_hashrate
3. eth_getWork
4. eth_submitWork
5. eth_submitHashrate

[lightclient]

I support this.

[mkalinin]

+1 from my side