MFC r345192-r345194: if_bridge(4): Drop pointless rtflush

r345192:
if_bridge(4): Drop pointless rtflush

At this point, all routes should've already been dropped by removing all
members from the bridge. This condition is in-fact KASSERT'd in the line
immediately above where this nop flush was added.

r345193:
Revert r345192: Too many trees in play for bridge(4) bits

An accidental appendage was committed that has not undergone review yet.

r345194:
if_bridge(4): Drop pointless rtflush

At this point, all routes should've already been dropped by removing all
members from the bridge. This condition is in-fact KASSERT'd in the line
immediately above where this nop flush was added.