Remove no longer used CI images

Author: kateinoigakukun

.github/workflows/ci-image.yml

@@ -15,22 +15,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - dockerfile: swift-ci/main/ubuntu/18.04/Dockerfile
-            platforms: linux/amd64,linux/arm64
-            tags:
-              - ghcr.io/swiftwasm/swift-ci:main-ubuntu-18.04
-          - dockerfile: swift-ci/main/ubuntu/20.04/Dockerfile
-            platforms: linux/amd64,linux/arm64
-            tags:
-              - ghcr.io/swiftwasm/swift-ci:main-ubuntu-20.04
           - dockerfile: swift-ci/main/ubuntu/22.04/Dockerfile
-            platforms: linux/amd64 # FIXME: linux/arm64 build didn't work on QEMU
-            tags:
-              - ghcr.io/swiftwasm/swift-ci:main-ubuntu-22.04
-          - dockerfile: swift-ci/main/amazon-linux/2/Dockerfile
             platforms: linux/amd64
             tags:
-              - ghcr.io/swiftwasm/swift-ci:main-amazon-linux-2
+              - ghcr.io/swiftwasm/swift-ci:main-ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
       - uses: docker/setup-buildx-action@v3