docker-util: Remove building of GCC 5 and 6 Docker images

Building of these images is no longer required as GCC 5 and 6 have been
dropped.

Change-Id: Ife0331843214fe545c5620a2f77ede3adb436700
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47462
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
diff --git a/util/cloudbuild/cloudbuild_create_images.yaml b/util/cloudbuild/cloudbuild_create_images.yaml
index 31876fb..7549012 100644
--- a/util/cloudbuild/cloudbuild_create_images.yaml
+++ b/util/cloudbuild/cloudbuild_create_images.yaml
@@ -20,20 +20,6 @@
     - name: 'gcr.io/cloud-builders/docker'
       args: ['build',
             '-t',
-            'gcr.io/$PROJECT_ID/gcc-version-5:latest',
-            '--build-arg', 'version=5',
-            'util/dockerfiles/ubuntu-18.04_gcc-version']
-
-    - name: 'gcr.io/cloud-builders/docker'
-      args: ['build',
-            '-t',
-            'gcr.io/$PROJECT_ID/gcc-version-6:latest',
-            '--build-arg', 'version=6',
-            'util/dockerfiles/ubuntu-18.04_gcc-version']
-
-    - name: 'gcr.io/cloud-builders/docker'
-      args: ['build',
-            '-t',
             'gcr.io/$PROJECT_ID/gcc-version-7:latest',
             '--build-arg', 'version=7',
             'util/dockerfiles/ubuntu-18.04_gcc-version']
@@ -110,8 +96,6 @@
 images: ['gcr.io/$PROJECT_ID/ubuntu-20.04_all-dependencies:latest',
          'gcr.io/$PROJECT_ID/ubuntu-20.04_min-dependencies:latest',
          'gcr.io/$PROJECT_ID/ubuntu-18.04_all-dependencies:latest',
-         'gcr.io/$PROJECT_ID/gcc-version-5:latest',
-         'gcr.io/$PROJECT_ID/gcc-version-6:latest',
          'gcr.io/$PROJECT_ID/gcc-version-7:latest',
          'gcr.io/$PROJECT_ID/gcc-version-8:latest',
          'gcr.io/$PROJECT_ID/gcc-version-9:latest',