ui-speedspacechart: remove dead code

The right-hand-side of the expression is never used, because a
non-empty string is always truthy.

Signed-off-by: Simon Ser <[email protected]>

Author: emersion

ui-speedspacechart/src/components/common/DetailsBox.tsx

@@ -90,7 +90,7 @@ const DetailsBox = ({
       {declivities && (
         <>
           <hr />
-          <span id="previous-gradient-text">{`${previousGradientText} ‰` || '--'}</span>
+          <span id="previous-gradient-text">{`${previousGradientText} ‰`}</span>
         </>
       )}
     </div>