Drop redundant code in `fail_holding_cell_htlcs`

Drop redundant code in `fail_holding_cell_htlcs`

`fail_holding_cell_htlcs` calls through to
`fail_htlc_backwards_internal` for HTLCs that need to be
failed-backwards but opts to generate its own payment failure
events for `HTLCSource:;OutboundRoute` HTLCs. There is no reason
for that as `fail_htlc_backwards_internal` will also happily
generate (now-)equivalent events for `HTLCSource::OutboundRoute`
HTLCs.

Thus, we can drop the redundant code and always call
`fail_htlc_backwards_internal` for each HTLC in
`fail_holding_cell_htlcs`.