bun_core: delete unused MapLike trait, FieldType marker, EmbedDir (#31543)

Follow-up to #31536 — removes the remaining unused comptime-reflection
placeholders from `util.rs` plus a few `bun_core` strays.

`cargo check --workspace` passes.

## Why this code exists

`src/bun_core/util.zig` is **not `@import`ed anywhere** in
`src/**/*.zig` (every `util` import points at `runtime/shell/util.zig`,
`node/util/validators.zig`, etc.; `bun.zig` does not re-export it). So
everything ported from that file was dead from day one.

| Item | Zig original | Zig callers | Status |
|---|---|---|---|
| `MapLike` trait | none — Zig used comptime duck-typing (`Map.KV`,
`@hasField`, `@FieldType` builtin) | n/a | **Rust-port artifact**
(translation of the comptime probes) |
| `Key<M>` / `Value<M>` | `src/bun_core/util.zig:3,7` | 0 — not even
used inside `util.zig` (`fromMapLike` calls the `@FieldType` builtin
directly) | **dead in Zig too** |
| `from_entries` | `util.zig:11` (`fromEntries`) | 0 external — only
called by `from()` at `:151,153`, which has 0 callers | **dead in Zig
too** |
| `from_map_like` | `util.zig:80` (`fromMapLike`) | 0 external — only
called by `from()` at `:139` | **dead in Zig too** |
| `FieldType` enum | `util.zig:101` (a `fn → ?type`) | 0 external — all
grep hits are the Zig builtin `@FieldType` or unrelated SQL enums |
**dead in Zig too** (and obsoleted by Zig's builtin `@FieldType`) |
| `EmbedDir` | none — neither `EmbedDir` nor `EmbedKind` exist in
`*.zig` | n/a | **Rust-port artifact** (alias for an alternate Rust-side
spelling) |
| `buffered_reader_size` | `src/bun_core/deprecated.zig:47`
(`bufferedReaderSize`) | 0 | **dead in Zig too** |
| `auto_format_label` / `_fallback` | `deprecated.zig:639,631` | 2 —
`src/runtime/node.zig:342`, `src/runtime/test_runner/expect.zig:122` |
Zig callers' Rust ports use native `Display`/`Debug` instead of comptime
format-label dispatch |
| `PathString::USE_SMALL_PATH_STRING` | `src/string/PathString.zig:12`
(`pub const use_small_path_string`) | 0 — `PathString.zig` only reads
the file-private `use_small_path_string_` (`:2`); the `pub` re-export
has no consumers | **dead in Zig too** (the public const; the private
backing value is still load-bearing) |

Also updates the stale port-note comment in `bundler/options.rs` that
pointed at the deleted `MapLike`.