Travis: Disable the macOS Rust job

This time, we're disabling it because it's the lowest priority job.
(And also slow.)

Part of 32629.

.travis.yml

@@ -62,10 +62,13 @@ matrix:
     - env: DISTCHECK="yes" ASCIIDOC_OPTIONS="" SKIP_MAKE_CHECK="yes"
     ## We check disable module dirauth
     - env: MODULES_OPTIONS="--disable-module-dirauth"
+    ## macOS builds are very slow, and we have a limited number of
+    ## concurrent macOS jobs. We're not actively developing Rust, so it is
+    ## the lowest priority.
     ## We run rust on macOS, because we have seen macOS rust failures before
-    - env: RUST_OPTIONS="--enable-rust --enable-cargo-online-mode"
-      compiler: clang
-      os: osx
+    #- env: RUST_OPTIONS="--enable-rust --enable-cargo-online-mode"
+    #  compiler: clang
+    #  os: osx
     ## We run chutney on macOS, because macOS Travis has IPv6
     - env: CHUTNEY="yes" CHUTNEY_ALLOW_FAILURES="2" SKIP_MAKE_CHECK="yes"
       os: osx