remove duplicate SKIPPED text

Currently, the output for skipped files looks like:

```
$ git commit...
Linting file: foo
SKIPPED: SKIPPED: no linter is defined or enabled for files with extension ""
```

where the first SKIPPED is in yellow.

This PR changes it to `SKIPPED: no linter is defined or enabled for files with extension ""`, where SKIPPED is still in yellow.