registry: no default inputs for Rust

We require users to have installed `cargo` (typically done via
`rustup`), which includes `rustc`, `cargo`, and `rustfmt` by default.

We could recommend `rustup` directly (e.g. instead of any distro
package) in order to easily support the `rust-toolchain.toml` file that
may or may not exist in any given project.

registry/registry.json

@@ -2,11 +2,7 @@
   "language": {
     "rust": {
       "default": {
-        "build-inputs": [
-          "rustc",
-          "cargo",
-          "rustfmt"
-        ]
+        "build-inputs": []
       },
       "dependencies": {
         "alsa-sys": {