The chosen token in the `send` flow was set from one of two places:
`metamask.selectedTokenAddress` or `metamask.send.token`. The former is
used most of the time, but the latter is used for the 'Edit' button
shown in the upper-left of the confirmation UI.
The send flow will now exclusively use `metamask.send.token` for the
token state during the send flow. `metamask.selectedTokenAddress` is
now only used for the selected token state on the Home screen. This
simplifies the Redux state, as the send token is now in one place
instead of two, and `metamask.selectedTokenAddress` has only one
purpose.
This function primarily ensured that there was a reasonable fallback
for `txParams` if the given transaction was missing it. This fallback
was only used in one place: the customize gas modal, during the send
flow specifically.
The helper function has been removed, and the default `txParams` is set
in the one place it was needed. In the future we should attempt to
simplify this modal so it doesn't need to know details about the
context in which it's being used.
