Add macros to remove old runtime

Summary:
This diff adds macros around the legacy architecture core.

To compile out the legacy architecture, simply set: -DRCT_FIT_RM_OLD_RUNTIME=1.

* RCTBridge: interface kept around
* RCTRootView: interface kept around
* RCTSurface: interface kept around
* RCTModuleData: interface kept around (used by RCTProfile)
* RCTProfile: Kept around (doesn't work in bridgeless...)
* RCTCxxBridge: interface kept around
* c++ bridge: removed
* legacy components in core: kept around (for now)

## Details
I added comments to each of the #else, and #endif directives. That way, we can more easily codemod this code in the future.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D72582307

fbshipit-source-id: 018d11cc488e97e60040bebf647f24f2437a57ce