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.
