This commit was added for equivalence with a similar fix to ip_gre.
That fix proved to have a bug. Upon closer inspection, ip6_gre is not
susceptible to the original bug.
So revert the unnecessary extra check.
In short, ipgre_xmit calls skb_pull to remove ipv4 headers previously
inserted by dev_hard_header. ip6gre_tunnel_xmit does not.