Remove the arm64 job from .travis.yml

According to

 https://docs.travis-ci.com/user/billing-overview/#partner-queue-solution

only ppc64le and s390x builds are still free for OSS builds, arm64
has been removed sometime in the course of last year. Thus drop this
job now to avoid to consume normal credits by accident here.

.travis.yml

@@ -18,10 +18,6 @@ script:
 matrix:
   include:
 
-    - env:
-        - CONFIG="--enable-werror --enable-debug"
-      arch: arm64
-
     - env:
         - CONFIG="--enable-werror --enable-debug"
       arch: ppc64le