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

This reverts commit 6f50175c.

Commit 183185a1 ("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>