Call setOverrideCssCode() once

There is no need to call this function each time a document is loaded.

Actually, setOverrideCssCode() was never called, because the slot
PhpDocumentationWidget::documentLoaded() is currently never invoked.
The reason is that porting kdev-php to QWidget-only
StandardDocumentationView was not finished. See disabled code in
PhpDocumentationWidget::PhpDocumentationWidget().