Removed the default "bridge" in builder

Now that the default network mode is "default" and this mode is chosen
even if the mode is empty string, it is not correct to have builder
still pointing to "bridge" as default (though this is daemon default).

Signed-off-by: Madhu Venugopal <madhu@docker.com>

builder/internals.go

@@ -569,7 +569,6 @@ func (b *Builder) create() (*daemon.Container, error) {
 		CgroupParent: b.cgroupParent,
 		Memory:       b.memory,
 		MemorySwap:   b.memorySwap,
-		NetworkMode:  "bridge",
 	}
 
 	config := *b.Config