chore(fish-speech): drop the darwin/metal build target (#10561)

The fish-speech metal-darwin-arm64 backend build has been failing on every
release (v4.5.3, v4.5.4, v4.5.5) and is a standing red on the darwin backend
matrix. fish-speech pulls `tokenizers` transitively from its upstream source
(`pip install -e fish-speech-src`), and on darwin/arm64 there is no prebuilt
wheel for the pinned old `tokenizers` version, so pip builds it from source.
Modern rustc rejects that old crate as a hard error:

    error: casting `&T` to `&mut T` is undefined behavior ...
       --> tokenizers-lib/src/models/bpe/trainer.rs:517:47
       = note: `#[deny(invalid_reference_casting)]` on by default
    error: could not compile `tokenizers` (lib) due to 1 previous error

This is deterministic, not a flake, and there is no clean fix that does not
either pin a stale Rust toolchain or downgrade a soundness lint guarding real
UB. Until upstream fish-speech moves to a tokenizers version that compiles on
current toolchains, drop darwin support so the release backend build stays
green. The Linux/CUDA/ROCm/Intel/L4T variants are unaffected.

Removes:
- the `-metal-darwin-arm64-fish-speech` entry from `includeDarwin` in
  backend-matrix.yml
- the `metal:` capability mappings and the concrete `metal-fish-speech` /
  `metal-fish-speech-development` gallery entries in backend/index.yaml
- the now-unused darwin-only requirements-mps.txt

Assisted-by: Claude:claude-opus-4-8 [Claude Code]

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
Co-authored-by: Ettore Di Giacinto <mudler@localai.io>

.github/backend-matrix.yml

@@ -4991,9 +4991,6 @@ includeDarwin:
   - backend: "qwen-tts"
     tag-suffix: "-metal-darwin-arm64-qwen-tts"
     build-type: "mps"
-  - backend: "fish-speech"
-    tag-suffix: "-metal-darwin-arm64-fish-speech"
-    build-type: "mps"
   - backend: "voxcpm"
     tag-suffix: "-metal-darwin-arm64-voxcpm"
     build-type: "mps"