sidecar channels: don't try to clean up previous batch state if in provider mode

[guggero]

If a node is providing sidecar channels, it won't be involved in the actual channel opening process.
So in case of a batch rollback (new PrepareMsg without a finalize of the previous batch), there is nothing to clean up.
In fact, the cleanup attempt creates log entries with errors:

2021-12-14 18:15:28.432 [INF] RPCS: Received PrepareMsg for batch=020xxxx, num_orders=1
2021-12-14 18:15:28.439 [WRN] FNDG: Unable to unregister funding shim (pendingChanID=2dxxxx) for order=8ff7xxx
2021-12-14 18:15:28.444 [DBG] FNDG: Cleaning up incomplete/replaced pending channel in lnd was unsuccessful for order=8ff7xxxx (channel_point=d3c5faxxxx:0), assuming timeout when funding: rpc error: code = Unknown desc = unable to find closed channel summary