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

This reverts commit 3380a4efd029c8836072e957b502df56ecbeefe2.

This is an attempt at fixing #504 now that the seamless reload
regtest that used to break non-x86 platforms was fixed.
diff --git a/.travis.yml b/.travis.yml
index d263cf7..fd136c9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -48,13 +48,6 @@
     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
     env: TARGET=linux-glibc OPENSSL_VERSION=1.1.1d COVERITY_SCAN_PROJECT_NAME="Haproxy" COVERITY_SCAN_BRANCH_PATTERN="*" COVERITY_SCAN_NOTIFICATION_EMAIL="chipitsine@gmail.com" COVERITY_SCAN_BUILD_COMMAND="make CC=clang TARGET=$TARGET $FLAGS 51DEGREES_SRC=$FIFTYONEDEGREES_SRC"