Dead code, stale comments and unused test inputs left by recent changes #8977

Found by differential semantic drift analysis from a7721303 to b123b7c3.

What is broken

Definitions, comments and test inputs left behind by changes in this range.

• .cmt code for the embedded .cmi that Stop writing a copy of the .cmi into .cmt and .cmti files #8774 stopped writing. Cmi_format.output_cmi (cmi_format.ml L64-L73) has no caller; the comment on create_cmi (L75-L77) still says save_cmt calls it; cmt_interface_digest is always None and never read. The nested read of a cmt after a cmi (cmt_format_common.ml L122-L130) is unreachable since the magic-number bumps in Remove OCaml leftovers #8805 and Remove OCaml attribute aliases and the features only OCaml used #8928, while cmt_format.mli L22-L27 and cmt_format_common.ml L110-L111 still promise that older files are read.
• Oprint.out_type_extension (oprint.ml L31-L32) is set by res_outcome_printer.ml L943 and read by nothing since Remove dead code #8756 removed its readers.
• Ext_namespace.try_split_module_name (ext_namespace.ml L38) is used only by its test since Remove dead code #8756 deleted outcome_printer_ns.ml.
• Ast_untagged_variants.block_type_can_be_undefined (L87-L91) has no caller since Match optional fields as options #8842; the module has no interface, so no warning reports it.
• Stale comment on Ast_helper.Jsx_component.take (ast_helper.ml L360-L364): an attribute "whose payload is not understood, is left in the attributes for the JSX transform to reject". Since Accept a payload on @react.componentWithProps with a deprecation warning #8944, a payload on @react.componentWithProps is accepted with a deprecation warning (jsx_common.ml L11-L38).
• Unused test inputs. tests/build_tests/regexp_statement_formatting/rescript.json (Keep regexp statements parseable after formatting #8720): the test runs bsc directly with explicit project root, package name and output, and passes without the file. rewatch-ocaml/tests/failure/Broken.invalid (Port rewatch to OCaml #8780) is identical to failure/src/Broken.res and named by no script; only Broken.fixed is read. The same holds for tests/build_tests/option_transitive_dependency/rescript.json (Fix compiler crash for options of transitive dependency types #8719, just before this range).
• Predating this range, for the same cleanup: (*TODO: add +10*) in bsc_warnings.ml L50 refers to a warning removed by Remove dead and unreachable error and warning variants #8459, and compiler/bsc/rescript_compiler_main.mli is an empty interface for an entry module that defines no value since Make compiler requests safe to run repeatedly and in parallel #8864.

Possible cause (speculation only)

A guess: the removals deleted the callers in the files they changed, and definitions exported through an interface (or in a module without one) produce no unused-value warning.