Don't verify miri's build status on beta

Author: alexcrichton

src/ci/docker/x86_64-gnu-tools/checktools.sh

@@ -31,7 +31,6 @@ python2.7 "$X_PY" test --no-fail-fast \
     src/doc/rust-by-example \
     src/tools/rls \
     src/tools/rustfmt \
-    src/tools/miri \
     src/tools/clippy
 set -e