The `SET_CURRENT_FIAT` action has been removed. It has been replaced
by a call to `forceUpdateMetamaskState`. The only purpose of this
action was to eagerly update the current fiat currency settings before
the next state update. Forcing a state update instead is simpler and
safer.
The `setCurrentCurrency` function in the background has been updated to
no longer return the state, now that it's no longer needed.
