rustdoc: remove redundant mobile-sized .source nav:not(.sidebar).sub
          #101397
        
          
      
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It's redundant because there's already a selector
.source nav.subwith exactly the same margin-left at line 796.This selector was added in 1e98fb1, along with an identical desktop selector, but that desktop selector was removed in 6a5f8b1 as part of a larger simplification.