fix(spec): remove invalid test

previous versions of normalize-css did not log an error message, with this version
error warnings are prited each time for invalid css.

after double checking the bug reports in slack and also the initial PR, we came to
the conclusion that we should default to print the error warning - especially as
there is no "silent" option in normalize-css available.

Co-authored-by: Björn Brauer <bjoern.brauer@new-work.se>

packages/spec/integration/postcss/styles.css

@@ -1,6 +1,5 @@
 @import url(imported-from-css.css);
 @import '@vishnucss/chips';
-@charset 'utf-8';
 
 .headline {
   composes: compose from './for-composing.css';