Remove UMD build of JS runtime library

We always use ESM, so we don't need to generate UMD runtime.js anymore.

Author: kateinoigakukun

Diff for: Makefile

@@ -22,6 +22,5 @@ unittest:
 .PHONY: regenerate_swiftpm_resources
 regenerate_swiftpm_resources:
 	npm run build
-	cp Runtime/lib/index.js Plugins/PackageToJS/Templates/runtime.js
 	cp Runtime/lib/index.mjs Plugins/PackageToJS/Templates/runtime.mjs
 	cp Runtime/lib/index.d.ts Plugins/PackageToJS/Templates/runtime.d.ts