openvswitch: remove dependency on SUPPORTED_KERNELS

openvswitch: remove dependency on SUPPORTED_KERNELS

Quote from Open vSwitch FAQ.md

    Open vSwitch userspace should also work with the Linux kernel module
    built into Linux 3.3 and later.

    Open vSwitch userspace is not sensitive to the Linux kernel version. It
    should build against almost any kernel, certainly against 2.6.32 and
    later.

The SUPPORTED_KERNEL dependency for openvswitch kernel module only
makes sense when we are building it from the ovs release tarballs
against mainline kernels.  Now that we are using the module from vanilla
kernel itself, the dependency does not exist anymore

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>