Remove dead code from the vendored Flow parser
Flow's parser moved to Rust, so the vendored OCaml copy (0.320.0) will never be synced again; it is now pruned like the rest of the repository and make dead-code no longer suppresses compiler/flow_parser.
Removed, about 3,700 lines:
- Unused functions, module-type items and collection helpers, and the [@@deriving show] debug printers. - Parser entry options ReScript never passes: parse options, the token sink and its rollback buffer. - File_key's variants: ReScript parses only strings, so the source of a location is always None and File_key.t is now empty. - Parse_error variants that are never raised. - AST fields the parser fills in but nothing reads: template literal quasi values (quasis are now locations), the records of tolerated syntax in record declarations and match cases (the tokens are still consumed), JSX text payloads and fragment locations, Comment.on_newline, and TSAccessibility comments.
Kept and marked live: Loc.equal, used by the derived Token.equal (ppx references are invisible to reanalyze), and the height field of the partial_node records cast from map and set nodes.