R1:34db543d2774

Remove all application-specific reply handler class overrides

Summary:
Ref T7199. In the vein of D12231, these options were a bad idea.

- They once served a very narrow, Facebook-specific need (see T1992), except even Facebook only used the Differential setting AFAIK.
- Outside of that special case, they are unused and essentially unusable (generally speaking, they do not meaningfully implement anything modular or replaceable).
- I have no knowledge of any install ever changing these settings, and…