Bluetooth: Mesh: Fix canceling publication retransmission timer

This branch is inherited from old code that used to start the timer as
soon as a message was queued for transmission rather than when it was
actually transmitted (the case today). It'll also cause a race in case
the publication goes over the GATT layer since the "sent" callback
happens synchronously in that case.

Fixes #17821

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>