Remove cmd-r override for rotation because it interferes with refresh (#5981)

# Description of Changes
Currently, cmd-r is set to rotate the PDF in the viewer instead of
perform refresh in the browser. This is unintuitive and confusing for
Mac users, and for Windows users (who are less used to doing ctrl-r for
refresh) it only works some of the time, if the Viewer is active, so
removing the override is no great loss.

frontend/src/core/components/viewer/EmbedPdfViewer.tsx

@@ -397,16 +397,6 @@ const EmbedPdfViewerContent = ({
               event.preventDefault();
               printActions.print();
               return;
-            case 'r':
-            case 'R':
-              // Ctrl+R: Rotate forward; Ctrl+Shift+R: Rotate backward
-              event.preventDefault();
-              if (event.shiftKey) {
-                rotationActions.rotateBackward();
-              } else {
-                rotationActions.rotateForward();
-              }
-              return;
           }
         }
       }