policy-node: fix potential rescan loop

SiLink activation might be delayed indefinitely under some error
conditions. Currently, policy-node schedules a rescan when it sees a
non-activated link on a stream to be moved, which produces busy loop if
the si-link doesn't activate.

Instead of rescheduling on non-active si-links, just remove and emit a
warning. The si-link then gets removed once it gets activated.

Reproducer:

1. Play audio from Rhythmbox and pause.
2. Switch default output with pactl between two different outputs
3. Links from the paused stream stay at "init"