QuickOpenLineEdit: don't set empty text and style sheet

8c8dbbdc removed setting of custom text and stylesheet from QuickOpenLineEdit::deactivate() in favor of KLineEdit::setClickMessage() (which was eventually ported to QLineEdit::setPlaceholderText()). But that commit failed to remove the text and stylesheet clearing counterparts from QuickOpenLineEdit::activate(). So now only activate() sets text and stylesheet to empty strings. Remove the pointless calls: deactivate() already clears text entered by the user, stylesheet is set nowhere else and thus is permanently empty anyway.