Remove unused legacy shared-component CSS

src/styles/annotator/components/_index.scss

 @use 'tailwindcss/components';
-@use '@hypothesis/frontend-shared/styles';
 
 // Annotator-specific components.
 @use './sidebar';

src/styles/sidebar/components/_index.scss

 @use 'tailwindcss/components';
 
-// Shared styles
-// -----------------
-@use '@hypothesis/frontend-shared/styles';
-
 // UI (Preact) Components
 // ----------
 @use './StyledText';