execute_next_action_full() logs that it removes the source when an
action returns ASYNC, but the code for that was incomplete, as it was
setting source_id to zero but not actually removing the source.
This lead to execute_next_action_full() being run continuously, only
alliviated by the default 10ms interval in the GSource from
`scenario->priv->action_execution_interval`.
This patch fixes that. As a drive-by fix it also adds locking to one
remaining unlocked usage of `priv->execute_actions_source_id`.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8014>
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