Remove lib/neutron-legacy leftovers

In [1] we finally got rid of the unfinished lib/neutron module and kept
only lib/neutron-legacy. It's renamed to lib/neutron now and it's the
only neutron related module in Devstack.
So this patch removes leftovers related to the old lib/neutron-legacy.

[1] https://review.opendev.org/c/openstack/devstack/+/865014

Change-Id: I698d87eba1fd92abfc4f631dc6109004d8bc827e

devstack/upgrade/upgrade.sh

@@ -41,7 +41,7 @@ set -o errexit
 source $TARGET_DEVSTACK_DIR/stackrc
 source $TARGET_DEVSTACK_DIR/lib/tls
 source $TARGET_DEVSTACK_DIR/lib/nova
-source $TARGET_DEVSTACK_DIR/lib/neutron-legacy
+source $TARGET_DEVSTACK_DIR/lib/neutron
 source $TARGET_DEVSTACK_DIR/lib/apache
 source $TARGET_DEVSTACK_DIR/lib/keystone
 source $TARGET_DEVSTACK_DIR/lib/database