system-macros: Don't explicitly remove bridge.

system-macros: Don't explicitly remove bridge.

Depending on the kernel in use, manually removing a bridge during
OVS_VSWITCHD_STOP can cause the kernel to send a route update that
refers to the bridge device which is being deleted. OVS can't make sense
of these messages, resulting in logs like the following:

route_table|DBG|Could not find interface name[702]: No such device or
address
netlink_notifier|WARN|received bad netlink message

One such example kernel is the Ubuntu 3.16.0-33-generic linux package.

The bridge doesn't need to be explicitly removed, as the OVSDB
configuration for each test will be cleared before executing the next.
Drop this extra step.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>