Move DiscardFunding generation into finish_close_channel

Move DiscardFunding generation into finish_close_channel

Currently the channel shutdown sequence has a number of steps which
all the shutdown callsites have to call. Because many shutdown
cases are rare error cases, its relatively easy to miss a call and
leave users without `Event`s or miss some important cleanup.

One of those steps, calling `issue_channel_close_events`, is rather
easy to remove, as it only generates two events, which can simply
be moved to another shutdown step.

Here we move the first of the two events, `DiscardFunding`, into
`finish_force_close_channel`.