rules/ remove run_under

rules/ were never meant to be added to the release. Now that we are not
*running* multi-arch binaries, we can remove the infrastructure that was
necessary to do so.

I may get back to this -- but we need to dust quite a bit off before
that happens.

ci/test

@@ -17,7 +17,7 @@ tools/bazel build "$@" \
     --experimental_use_hermetic_linux_sandbox \
     --sandbox_writable_path="$cache_prefix" \
     --sandbox_add_mount_pair=/proc \
-    //test/c:which_libc_linux_amd64_gnu.2.19
+    //test/c:which_libc
 
 # then test everything else with the standard sandbox
 echo "--- bazel test $* ..."