PERF: Remove unneeded recursive `chown` (#567)

PERF: Remove unneeded recursive `chown` (#567)

This command can take a very long time (> 2 minutes on a CDCK build machine) now that the directory contains the `yarn` cache. However, there are no files in `/home/discourse` that are owned by a different user, so the command does absolutely nothing. This can be demonstrated by using the `-c` flag (which prints any changes made):

```
docker run --rm -it discourse/base:2.0.20210826-1706 /bin/bash -c "time chown -cR discourse /home/discourse"
```

This has an empty output for the latest base image. Therefore this line can be safely removed