bgpd: clear max prefix overflow on de-config

bgpd: clear max prefix overflow on de-config

A bgp neighbor remains in Idle state in the event that the number
of received prefixes exceeds the configured maximum prefix for the
neighbor. The neighbor remains in idle state even after de-configuring
the maximum prefix limit for the neighbor.

The fix is to clear the neighbor overflow state if set, after
de-configuring the neighbor maximum-prefix commnd.

This allows the neighbor to establish without having to perform a
clear operation. It also avoids the misleading neigbor summary
indicating that the neighbor is in prefix overflow state (PfxCt)
when no limit is configured for the neighbor.

Signed-off-by: Dewi Morgan <dewi.morgan@intl.att.com>