net: ip6_gre: Remove the unused function ip6gre_tnl_addr_conflict()

net: ip6_gre: Remove the unused function ip6gre_tnl_addr_conflict()

The function ip6gre_tnl_addr_conflict() is defined in the ip6_gre.c file,
but not called elsewhere, so delete this unused function.

net/ipv6/ip6_gre.c:887:20: warning: unused function 'ip6gre_tnl_addr_conflict'.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2419
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221017093540.26806-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>