Remove our “rpc” package (#481)

As far as I can tell it has been completely superseded by [comlink](https://github.com/GoogleChromeLabs/comlink).

We also don’t use it in Webviz directly since we have our own RPC
implementation; we basically forgot to switch over to this package. I
also couldn’t find a reference to this package in any Cruise codebase.

If anyone still uses this they should just switch over to comlink. It
might be nice to do that in Webviz eventually too, but there’s no rush,
we can keep our local Rpc implementation for now.

.eslintrc.js

@@ -17,7 +17,6 @@ module.exports = {
     "import/external-packages": [
       "packages/@cruise-automation/button",
       "packages/@cruise-automation/hooks",
-      "packages/@cruise-automation/rpc",
       "packages/@cruise-automation/tooltip",
       "packages/react-key-listener",
       "packages/regl-worldview",