[ruby/fiddle] Remove needless workaround

It's fixed in upstream.
https://github.com/MSP-Greg/ruby-loco/issues/4

https://github.com/ruby/fiddle/commit/2ae0ff4934

ext/fiddle/fiddle.gemspec

@@ -66,7 +66,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rake"
   spec.add_development_dependency "rake-compiler"
 
-  # ruby head for mingw uses sub!:
-  # https://github.com/MSP-Greg/ruby-loco/blob/master/patches/__operating_system.rb.patch
-  spec.metadata["msys2_mingw_dependencies"] = +"libffi"
+  spec.metadata["msys2_mingw_dependencies"] = "libffi"
 end