Remove fetch API

[enricobottazzi]

The latest update of summa, after #141 is merged, will support any cryptocurrencies with no difference or preference.

Therefore, the fetch API should be able to fetch balances from any blockchain at any block. The current backend implementation only partially supports it => now only able to fetch ETH (at any block) and ERC20 (only at the most recent block).

I believe that removing the feature as a whole is better than having a partially functioning feature.

[sifnoc]

This issue can be resolved following #152
The related commit can be found 9a93558