Bug 1487103: Remove useless .catch(). r=trivial

toolkit/components/extensions/parent/ext-runtime.js

@@ -89,9 +89,7 @@ this.runtime = class extends ExtensionAPI {
               fire.sync(details);
             });
             return () => {
-              AddonManager.removeUpgradeListener(instanceID).catch(e => {
-                // This can happen if we try this after shutdown is complete.
-              });
+              AddonManager.removeUpgradeListener(instanceID);
             };
           },
         }).api(),