Delete dead code in aec_chain_state
Delete dead code in aec_chain_state

* The deleted clause can be reached only by call
  `update_main_chain(get_genesis_hash(State), NewTopHash, State)`.

* Conductor stores genesis block at initialization.

* Function `update_main_chain` is called only by conductor.

* Conductor calls `maybe_add_genesis_hash` on state before calling
  `update_main_chain`.

Hence the deleted clause can be reached only if:

* Conductor init is buggy i.e. initializes DB with "genesis" block
  that has not genesis height;

* The DB is corrupted.

aeternity/epoch,aeternity/epoch,aeternity/epoch,aeternity/epoch