The new approach uses a dev-mode-only CLJC reader conditional
`#?(:cljs-dev ...)` to make `metabase.util.devtools` empty
except in dev builds.

The previous approach for this relied on dead code elimination.
But DCE relies on tracing function calls and `:require`s from the
`:entries` list in shadow-cljs.edn. Top-level namespace code is
handled differently; in particular a namespace listed in `:entries`
like `metabase.util.devtools` will always have its top-level code
in the build.