Revert "selftests: Add IPv6 link-local address generation tests for GRE devices."

This reverts commit 6f50175.

Commit 183185a ("gre: Fix IPv6 link-local address generation.") is
going to be reverted. So let's revert the corresponding kselftest
first.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/259a9e98f7f1be7ce02b53d0b4afb7c18a8ff747.1742418408.git.gnault@redhat.com
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Guillaume Nault

tools/testing/selftests/net/Makefile

@@ -31,7 +31,6 @@ TEST_PROGS += veth.sh
 TEST_PROGS += ioam6.sh
 TEST_PROGS += gro.sh
 TEST_PROGS += gre_gso.sh
-TEST_PROGS += gre_ipv6_lladdr.sh
 TEST_PROGS += cmsg_so_mark.sh
 TEST_PROGS += cmsg_so_priority.sh
 TEST_PROGS += cmsg_time.sh cmsg_ipv6.sh