Revert "ci: include s390x release binaries (ggml-org#16648)"

This reverts commit fcb235b.

Author: yairpatch

.github/workflows/release.yml

@@ -134,8 +134,6 @@ jobs:
         include:
           - build: 'x64'
             os: ubuntu-22.04
-          - build: 's390x-z15' # z15 because our CI runners are on z15
-            os: ubuntu-22.04-s390x
           # GGML_BACKEND_DL and GGML_CPU_ALL_VARIANTS are not currently supported on arm
           # - build: 'arm64'
           #   os: ubuntu-22.04-arm