[router-table] fix signalling when neighbor routers are removed (#6948)
[router-table] fix signalling when neighbor routers are removed (#6948)

This commit updates `RouterTable` to ensure signalling removal of
neighbor routers when router table is fully cleared or when a
`Router` with a specific released router ID is cleared.
abtink/openthread,jwhui/openthread,srickardti/openthread,abtink/openthread,jwhui/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,openthread/openthread,openthread/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread