Remove kernel version check for OVS VXLAN

Upstream kernel version check for OVS VXLAN doesn't work
in RHEL, because we backport those features to our 2.6.x
kernel branch. The check is removed downstream until
a solution that works for everyone is available.

Resolves rhbz: 1100164
Downstream-Only
Upstream-Juno: https://review.openstack.org/#/c/88121/

Change-Id: Ica9188c624bf8f7ba91eee2537a2219a97017b54
Reviewed-on: https://code.engineering.redhat.com/gerrit/25694
Tested-by: RHOS Jenkins <apevec+rhosci@redhat.com>
Reviewed-by: Miguel Angel Ajo Pelayo <majopela@redhat.com>
Reviewed-by: Ihar Hrachyshka <ihrachys@redhat.com>
Tested-by: Ihar Hrachyshka <ihrachys@redhat.com>