util-docker: Remove GCC Version 9 from Dockerfiles

As we are no longer testing for GCC Version 9, we no longer need to
compile these Docker images.

Change-Id: Ia8fc712043ce211ff46da47fdce691a67ecdbb54

util/dockerfiles/docker-compose.yaml

@@ -44,13 +44,6 @@ services:
             args:
                 - version=8
         image: gcr.io/gem5-test/gcc-version-8:latest
-    gcc-9:
-        build:
-            context: ubuntu-20.04_gcc-version
-            dockerfile: Dockerfile
-            args:
-                - version=9
-        image: gcr.io/gem5-test/gcc-version-9:latest
     gcc-10:
         build:
             context: ubuntu-20.04_gcc-version

util/dockerfiles/ubuntu-20.04_gcc-version/Dockerfile

@@ -28,7 +28,7 @@ FROM --platform=${BUILDPLATFORM} ubuntu:20.04
 # Valid version values:
 # 7
 # 8
-# 9
+# 9 # Not supported. See: https://github.com/gem5/gem5/issues/555.
 # 10
 ARG version