system contract cleanup: remove special cases of eosio delegate/buy

[moskvanaft]

Previously we always transfered tokens paid for bandwidth and ram to eosio account. And because of that we had special handling for cases when eosio is paying for ram and bandwidth (in order not to transfer to itself). It was somewhat tricky because eosio didn't pay anything, and could even stake for bandwidth more than total token supply. Although it's unlikely to affect anything except our unit-tests.

Now we transfer money to separate accounts. So, there is no reason to keep special handling for eosio. So, I am removing those checks. In order to simplify the code.