CI: travis-ci: temporarily disable arm64 builds

few recent builds failed with "gcc: fatal error: Killed signal
terminated program cc1", let us disable arm64 builds until investigation
diff --git a/.travis.yml b/.travis.yml
index 3083e30..7f5110e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,10 +22,10 @@
     arch: ppc64le
     compiler: gcc
     if: type == cron
-  - os: linux
-    arch: arm64
-    compiler: gcc
-    if: type == cron
+#  - os: linux
+#    arch: arm64
+#    compiler: gcc
+#    if: type == cron
   - os: linux
     arch: arm64-graviton2
     group: edge