Avoid redundant broadcast of local commitment transaction

Avoid redundant broadcast of local commitment transaction

This change follows the rationale of commit 62236c7 and addresses the
last remaining redundant local commitment broadcast.

There's no need to broadcast our local commitment transaction if we've
already seen a confirmed one as it'll be immediately rejected as a
duplicate/conflict.

This will also help prevent dispatching spurious events for bumping
commitment and HTLC transactions through anchor outputs since the
dispatch for said events follows the same flow as our usual commitment
broadcast.