i40e: remove CONFIG_I40E_VXLAN

Instead of having our own custom symbol, we can just rely
on whether or not the kernel has the feature enabled.

In this case use IS_ENABLED(CONFIG_VXLAN) in order to handle
built-in or module in the current BKM way.

Change-ID: I5890fbb518ff8ed6bb07c3362fb0a8a829f9b241
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>