Don't pass --quiet to csslint

Summary: Similar to D11198. This flag doesn't do anything when combined with `--format=lint-xml`.

Test Plan:
```lang=bash
> csslint *.css
csslint *.css

csslint: There are 1 problems in /home/joshua/workspace/github.com/phacility/arcanist/fail.css.

fail.css
1: error at line 1, col 1
Unexpected token '~' at line 1, col 1.
~

csslint: No errors in /home/joshua/workspace/github.com/phacility/arcanist/pass.css.

> csslint --quiet *.css
csslint --quiet *.css

csslint: There are 1 problems in /home/joshua/workspace/github.com/phacility/arcanist/fail.css.

fail.css
1: error at line 1, col 1
Unexpected token '~' at line 1, col 1.
~

> csslint --format=lint-xml *.css
<?xml version="1.0" encoding="utf-8"?><lint>
<file name="/home/joshua/workspace/github.com/phacility/arcanist/fail.css"><issue line="1" char="1" severity="error" reason="Unexpected token '~' at line 1, col 1." evidence="~"/></file>
</lint>

> csslint --format=lint-xml --quiet *.css
<?xml version="1.0" encoding="utf-8"?><lint>
<file name="/home/joshua/workspace/github.com/phacility/arcanist/fail.css"><issue line="1" char="1" severity="error" reason="Unexpected token '~' at line 1, col 1." evidence="~"/></file>
</lint>
```

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11308

src/lint/linter/ArcanistCSSLintLinter.php

@@ -28,7 +28,6 @@ final class ArcanistCSSLintLinter extends ArcanistExternalLinter {
   protected function getMandatoryFlags() {
     return array(
       '--format=lint-xml',
-      '--quiet',
     );
   }