fix: Don't attempt to remove an unmapped file.

Quite frequently, I'm seeing `fileMap[filepath]` be undefined in `DependencyResolver.removeFile()`. I run this in IntelliJ and it seems to happen when I'm saving quite frequently.

I'm not really sure what the root cause is here, so this is a bit of a bandaid. When using IntelliJ I see it trying to remove files such as /Users/rob/.../tests/integration/TrackingIntegrationTests.js___jb_old___ even though I've configured just to map *.js.