datapath: Remove WARN_ON_ONCE(1) now that this code has been exercised.

datapath: Remove WARN_ON_ONCE(1) now that this code has been exercised.

The code on one side of this #if fork was difficult to test until Xen
upgraded to a new enough kernel that it would exercise it.  Later Xen
kernels are now available and this code path has been tested, at least to
some extent, so remove the warning.

Thanks to Ian Campbell <Ian.Campbell@citrix.com> for pointing out the
warning.