Remove stack trace when lport migration is missing
Remove stack trace when lport migration is missing

Lport migration is checked every time a port is deleted. Lport migration
missing is a perfectly fine condition, this patch demotes the log to
DEBUG level.

Closes-Bug: #1691903
Change-Id: I3975cd7eddc106b817b529fac2994e00f02d7903

openstack/dragonflow,openstack/dragonflow,openstack/dragonflow