use EGRESS_DIRECTION and INGRESS_DIRECTION from neutron-lib

The EGRESS_DIRECTION and INGRESS_DIRECTION constants live in neutron-lib
now and neutron is going to consume them in
I1b81f5c3de9e6f2c0967c2db23ddb716ee7ec6b9

This patch removes the EGRESS_DIRECTION, INGRESS_DIRECTION and
VALID_DIRECTIONS constants from common.constants as they are already
defined and use from neutron-lib.

Change-Id: I52841ecd716f6dbf87fc4c5543d68ba378edbeea
1 file changed