powerpc/ci: Drop Ubuntu 18.04 perf build

The 18.04 perf build is failing, and has been dropped by upstream perf:
  acmel/linux-tools-container-builds@8ee261d

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Author: mpe

.github/workflows/powerpc-perf.yml

@@ -27,7 +27,7 @@ jobs:
 
     strategy:
       matrix:
-        image: [ubuntu-22.04, ubuntu-20.04, ubuntu-18.04]
+        image: [ubuntu-22.04, ubuntu-20.04]
         subarch: [ppc64, ppc64le]
 
     env: