Revert "Add IBM Z to Travis configuration (#42)"

This reverts commit 7972beb375.

.travis.yml

@@ -34,16 +34,6 @@ matrix:
         - TARGET_BOX=PPC64LE_LINUX
         - BTYPE="BINARY=64 USE_OPENMP=1"
 
-    - <<: *test-ubuntu
-      os: linux
-      arch: s390x
-      before_script:
-        - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=Z13 NUM_THREADS=32"
-      env:
-        # for matrix annotation only
-        - TARGET_BOX=IBMZ_LINUX
-        - BTYPE="BINARY=64 USE_OPENMP=1"
-
     - <<: *test-ubuntu
       env:
         - TARGET_BOX=LINUX64