The `accounts` prop of `SignatureRequest` was throwing a PropType
warning because `accounts` was an object instead of an array. It looks
like when the `mergeProps` function was added in #6340, the ownProps
were accidentally set to override the state props.
The now ignored props have been removed from the parent `ConfirmTxScreen`
component as well. `conversionRate` was identical to the one retrieved
in `SignatureRequest`, and `selectedAddress` differed only in the
fallback behaviour when `state.metamask.selectedAddress` does not exist;
it will now default to the first account instead (as was the original
behavior, prior to #6340).
