Stop registering a Go SDK in the Gazelle submodule (#411)

* Do not register a Go toolchain for ourselves

This module can be used as a transitive dep, and so we want to
avoid forcing users to futz with their SDKs.

* Add a small comment

gazelle/MODULE.bazel

@@ -9,16 +9,7 @@ bazel_dep(name = "bazel_skylib", version = "1.3.0")
 bazel_dep(name = "gazelle", repo_name = "bazel_gazelle", version = "0.28.0")
 bazel_dep(name = "rules_go", repo_name = "io_bazel_rules_go", version = "0.35.0")
 
-go_sdk = use_extension("@io_bazel_rules_go//go:extensions.bzl", "go_sdk")
-
-go_sdk.download(
-    name = "go_sdk",
-    version = "1.18",
-)
-
-use_repo(go_sdk, "go_sdk")
-
-register_toolchains("@go_sdk//:all")
+# `rules_go` will register a toolchain for us if the user doesn't do so
 
 go_deps = use_extension("@bazel_gazelle//:extensions.bzl", "go_deps")