Update editor_iequirks.css

Hi, how about to replace this line to valid css? As it were made here https://github.com/ckeditor/ckeditor-dev/commit/e5555ee210a9761e8f2fc8bd7993241ccf555056

I always need to support my branch, because of Rails precompiler fails with error
```
+Invalid CSS after "    filter: ": expected expression (e.g. 1px, bold), was ";"
```