test_bgp_neighbor_af: add flush
test_bgp_neighbor_af: add flush

* Two tests fail when we run `rake test SEED='19450'` on the entire minitest suite
 * Both tests create nbr_af's in the default vrf, then one more in vrf 'aa'
 * Debugging showed that 'aa' AF exists on device, but RouterBgpNeighborAF.afs only finds the first two nbr_af's
 * cache_flush was the only way to get it to show up with afs (sleep and config() had no effect)
 * problem does not occur when we run test_bgp_neighbor_af.rb standalone

* Removed the 'no feature bgp' calls since setup & teardown are already doing `remove_all_bgps`
* Tested on n9k-I3

chrisvanheuveln/cisco-network-node-utils,chrisvanheuveln/cisco-network-node-utils,cisco/cisco-network-node-utils,cisco/cisco-network-node-utils