[cleanup] remove yggs part 1/2 (bifrost)
Breaking bifrost out from !3155 (merged). It has a decent amount of work/changes on its own and the memo can't be disallowed in thornode without these bifrost updates.
Note that vaultTransfer events are only triggered by yggs, so that logic is removed (should be removed from the solidity at some point as well).
Edited by akrokr