Remove unused .csslintignore

This file was for the Jenkins job of csslint (since it doesn't
use the Gruntfile), now that it does, this file is redundant.

See also 94b213bd1ec33a2 in oojs-ui.

Change-Id: I62c2eb9cf62ebb15a19158f4913bf8a18597c7d4
Merge "Remove unused .csslintignore"