Remove unnecessary imports for RCTHTTPRequestHandler to fix Cocoapods build

Summary:
@tadeuzagallo - We discussed this ~a week ago when I was putting together the 0.7.0-rc release, only got around to creating the PR for it now so it's properly sync'd.
Closes https://github.com/facebook/react-native/pull/1865
Github Author: Brent Vatne <brentvatne@gmail.com>

Libraries/Network/RCTHTTPRequestHandler.m

@@ -9,13 +9,6 @@
 
 #import "RCTHTTPRequestHandler.h"
 
-#import "RCTAssert.h"
-#import "RCTConvert.h"
-#import "RCTEventDispatcher.h"
-#import "RCTImageLoader.h"
-#import "RCTLog.h"
-#import "RCTUtils.h"
-
 @interface RCTHTTPRequestHandler () <NSURLSessionDataDelegate>
 
 @end