Revert^4 "add rust_toolchain_rustc_prebuilt module type"

930fd8bfb1190e20789c332d3f8d7de79bcee13c

Change-Id: Id3f3a66022a496aab7db93670a7bdf8507384875
diff --git a/rust/config/global.go b/rust/config/global.go
index c976617..f397ce9 100644
--- a/rust/config/global.go
+++ b/rust/config/global.go
@@ -81,13 +81,7 @@
 
 func init() {
 	pctx.SourcePathVariable("RustDefaultBase", RustDefaultBase)
-	pctx.VariableConfigMethod("HostPrebuiltTag", func(config android.Config) string {
-		if config.UseHostMusl() {
-			return "linux-musl-x86"
-		} else {
-			return config.PrebuiltOS()
-		}
-	})
+	pctx.VariableConfigMethod("HostPrebuiltTag", HostPrebuiltTag)
 
 	pctx.VariableFunc("RustBase", func(ctx android.PackageVarContext) string {
 		if override := ctx.Config().Getenv("RUST_PREBUILTS_BASE"); override != "" {
@@ -109,6 +103,14 @@
 	exportedVars.ExportStringStaticVariable("RUST_DEFAULT_VERSION", RustDefaultVersion)
 }
 
+func HostPrebuiltTag(config android.Config) string {
+	if config.UseHostMusl() {
+		return "linux-musl-x86"
+	} else {
+		return config.PrebuiltOS()
+	}
+}
+
 func getRustVersionPctx(ctx android.PackageVarContext) string {
 	return GetRustVersion(ctx)
 }