Roll out RCTNetworking extraneous NativeModule call removal

Summary:
## Context
Every time we call RCTNetworking.sendRequest(), we [set up six event listeners inside XMLHttpRequest](https://fburl.com/diffusion/85k6ou5w) by calling RCTNetworking.addListener(). Seeing how RCTNetworking.addListener() is implemented, each call results in two async NativeModule call: [one to addListener()](https://fburl.com/diffusion/ng21jek6), and [another to removeEventListener()](https://fburl.com/diffusion/nua3y973).

For RCTNetworking, both of these NativeModule calls are unnecessary, as explained in D24272663 (dabca52)
> RCTNetworking.startObserving and RCTNetworking.stopObserving don't exist. The main purpose of RCTEventEmitter.addListener is to call these methods, and increment the _listeners counter, so that we can start dispatching events when _listeners > 0. In D24272560 (82187bf), I made RCTEventEmitter dispatch events even when _listeners <= 0. This is sufficient for us to stop calling these two RCTNetworking methods entirely.

Therefore, this experiment gets rid of on average 6-8 NativeModule method calls for every network call we make in React Native on iOS.

Reviewed By: PeteTheHeat

Differential Revision: D25618704

fbshipit-source-id: 0da20475a0882ed737cf32de27f266fd2cd016af

Author: RSNara

Libraries/Network/RCTNetworking.ios.js

@@ -18,13 +18,8 @@ import type {RequestBody} from './convertRequestBody';
 
 class RCTNetworking extends NativeEventEmitter {
   constructor() {
-    const disableCallsIntoModule =
-      typeof global.__disableRCTNetworkingExtraneousModuleCalls === 'function'
-        ? global.__disableRCTNetworkingExtraneousModuleCalls()
-        : false;
-
     super(NativeNetworkingIOS, {
-      __SECRET_DISABLE_CALLS_INTO_MODULE_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: disableCallsIntoModule,
+      __SECRET_DISABLE_CALLS_INTO_MODULE_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: true,
     });
   }