Remove the Scribe.noop logger

[henrylee97]

Summary

Removes the prebuilt Scribe.noop logger from the core library so Scribe ships only the essential building blocks of value-based structured logging (levels, fields, events, sinks, loggers).

A noop logger is a convenience value users can assemble in a line when they actually need one, so it doesn't need to live in the core API:

Scribe.create ~level:Scribe.Level.Debug ~sink:(Scribe.Sink.make (fun _ -> ()))
(* or, using the concrete sink package *)
Scribe.create ~level:Scribe.Level.Debug ~sink:(Scribe_sinks.Noop.create ())

Changes

• lib/scribe.ml — delete the let noop = … definition.
• lib/scribe.mli — remove val noop : t and its doc comment; reword the module-level doc comment that referenced [noop] so it no longer dangles.
• test/test_scribe.ml — delete test_noop and its "noop logger" registration.

The Scribe_sinks.Noop sink is intentionally left unchanged — it's a concrete sink in the separate scribe.sinks package (not a logger), and it's exactly the primitive used to rebuild a noop logger. Its "noop sink" test stays.

Verification

• dune build @fmt — formatting passes
• dune build — compiles, no remaining Scribe.noop references
• dune runtest — 4/4 tests pass (including noop sink)
• dune build @install — passes