iOS changes to switch to JSI

Summary:
change RCTCxxBridge to use JSIExecutorFactory+JSCRuntime
instead of JSCExecutorFactory.  Also remove JSC usage from RN in other
files.  This allows deleting files, too, which is done further down
the stack.

Reviewed By: RSNara

Differential Revision: D9369111

fbshipit-source-id: 67ef3146e3abe5244b6cf3249a0268598f91b277

React/Base/RCTJavaScriptLoader.mm

@@ -10,7 +10,6 @@
 #import <sys/stat.h>
 
 #import <cxxreact/JSBundleType.h>
-#import <jschelpers/JavaScriptCore.h>
 
 #import "RCTBridge.h"
 #import "RCTConvert.h"
@@ -20,6 +19,8 @@
 
 NSString *const RCTJavaScriptLoaderErrorDomain = @"RCTJavaScriptLoaderErrorDomain";
 
+static const int32_t JSNoBytecodeFileFormatVersion = -1;
+
 @interface RCTSource()
 {
 @public