[ReactNative] Remove unused executor context id

Summary:
Remove `RCTGetExecutorID` and `RCTSetExecutorID`, it wasn't used anymore since
the bridge was refactored into `RCTBridge` and `RCTBatchedBridge`.

React/Base/RCTJavaScriptExecutor.h

@@ -36,7 +36,6 @@ typedef void (^RCTJavaScriptCallback)(id json, NSError *error);
 - (void)executeJSCall:(NSString *)name
                method:(NSString *)method
             arguments:(NSArray *)arguments
-              context:(NSNumber *)executorID
              callback:(RCTJavaScriptCallback)onComplete;
 
 /**
@@ -66,6 +65,3 @@ typedef void (^RCTJavaScriptCallback)(id json, NSError *error);
 - (void)executeAsyncBlockOnJavaScriptQueue:(dispatch_block_t)block;
 
 @end
-
-void RCTSetExecutorID(id<RCTJavaScriptExecutor> executor);
-NSNumber *RCTGetExecutorID(id<RCTJavaScriptExecutor> executor);