selftests: net: remove unneeded IP_GRE config

It seems that there is no definition for config IP_GRE, and it is not a
dependency of other configs, so remove it.

linux$ find -name Kconfig | xargs grep "IP_GRE"
<-- nothing

There is a IPV6_GRE config defined in net/ipv6/Kconfig. It only depends
on NET_IPGRE_DEMUX but not IP_GRE.

Signed-off-by: Yujie Liu <yujie.liu@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20240624055539.2092322-1-yujie.liu@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>