[ReactNative] Remove RCT_IMPORT_METHOD macro and generate lookup table dynamically

Summary:
@public

This removes the last piece of data that was still stored on the DATA section,
`RCT_IMPORT_METHOD`. JS calls now dynamically populate a lookup table simultaneously
on JS and Native, instead of creating  a mapping at load time.

Test Plan: Everything still runs, tests are green.

React/Modules/RCTTiming.m

@@ -74,8 +74,6 @@
 
 RCT_EXPORT_MODULE()
 
-RCT_IMPORT_METHOD(JSTimersExecution, callTimers)
-
 - (instancetype)init
 {
   if ((self = [super init])) {