jest: Comment about boring mocks, remove one that's unnecessary.

I'm not sure when exactly it became unnecessary to mock "Linking",
but it's taken care of now in React Native's Jest setup [1].

[1]: https://github.com/facebook/react-native/blob/v0.61.5/jest/setup.js#L153

jest/jestSetup.js

@@ -25,9 +25,9 @@ jest.mock('react-native', () =>
   // Extend ReactNative
   Object.setPrototypeOf(
     {
-      // Adding something directly from RN here, as in
-      // `Foo: ReactNative.Foo`, is not done because we want to mock
-      // Foo ourselves.
+      // Boring RN mocks. Adding something directly from RN here, as
+      // in `Foo: ReactNative.Foo`, is not done because we want to
+      // mock Foo ourselves.
       //
       // One reason to have to do this is because of an odd
       // indirection in `react-native-vector-icons`. They re-export
@@ -43,19 +43,27 @@ jest.mock('react-native', () =>
       Platform: ReactNative.Platform,
       StyleSheet: ReactNative.StyleSheet,
       NativeModules: ReactNative.NativeModules,
-
-      Linking: {
-        addEventListener: jest.fn(),
-        removeEventListener: jest.fn(),
-        openURL: jest.fn(),
-        canOpenURL: jest.fn(),
-        getInitialURL: jest.fn(),
-      },
     },
     ReactNative,
   ),
 );
 
+/**
+ * Boring mocks
+ *
+ * We aren't interested in any specific data in these mocks; they just
+ * make things not break. Usually it's because these JS modules depend
+ * on something being available on NativeModules, and we'd rather not
+ * mock that precisely, as it's an implementation detail that may
+ * change.
+ *
+ * If Jest complains about syntax errors in a module in node_modules,
+ * it's likely because the only code it finds for the module is modern
+ * JavaScript that needs to be transformed by Babel. In that case, try
+ * adding the module to `transformModulesWhitelist` in our Jest config
+ * before mocking it here.
+ */
+
 jest.mock('@react-native-community/async-storage', () => mockAsyncStorage);
 
 jest.mock('react-native-sound', () => () => ({