Revert "BUILD: travis-ci: enable s390x builds"

This reverts commit 3380a4efd0.

This is an attempt at fixing #504 now that the seamless reload
regtest that used to break non-x86 platforms was fixed.

.travis.yml

@@ -47,13 +47,6 @@ matrix:
     if: type != cron
     compiler: clang
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
-  - os: linux
-    arch: s390x
-    if: type != cron
-    compiler: gcc
-    env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d
-    before_script:
-      - rm reg-tests/seamless-reload/abns_socket.vtc # please, see https://github.com/haproxy/haproxy/issues/504
   - os: linux
     if: type == cron
     compiler: clang