Don't use for GCC<9

RHEL-8 system compiler is the last GCC 8 we still test. Upstream kernel
building on aarch64 has been broken since [1] and apparently it's not
just a short temporary blip. Considering the maintenance only status
of RHEL-8 system compiler, let's just stop using this test there.

[1] https://lore.kernel.org/lkml/CA+G9fYsHGrgZsEEVvP0XMcAhLyCYnrCPgZJ1puT6cfQBCGUB9g@mail.gmail.com/T/#m089a8e13fdc7934455312a72c9bd62ae8d40332a

tests/Sanity/scm-test/main.fmf

@@ -14,6 +14,9 @@ adjust+:
   - enabled: false
     when: distro == rhel-8 and distro ~< rhel-8.3
     continue: false
+  - enabled: false
+    when: distro == rhel-8 and collection is not defined
+    continue: false
 contact: Vaclav Kadlcik <vkadlcik@redhat.com>
 component:
   - gcc