build: simplify rust toolchain module

AGENTS.md

@@ -34,3 +34,4 @@
 
 - When materializing an external specification, standard, or project-maintained convention, use the upstream source instead of reconstructing it from memory.
   Preserve local policy separately from copied upstream text.
+- After an explicit squash or rewrite request on a pull request, subsequent pull-request updates still use follow-up commits unless the user explicitly asks for another rewrite.

MODULE.bazel

@@ -2,12 +2,7 @@ module(name = "alexion", version = "0.1.0")
 
 bazel_dep(name = "rules_rust", version = "0.74.0")
 
-rust = use_extension("@rules_rust//rust:extensions.bzl", "rust")
-rust.toolchain(versions = [])
-use_repo(rust, "rust_toolchains")
-
 local_rust = use_extension("//bazel:rust_toolchain.bzl", "local_rust")
 use_repo(local_rust, "local_rust_toolchain")
 
-register_toolchains("@rust_toolchains//:all")
 register_toolchains("@local_rust_toolchain//:local_rust_toolchain")