Drop x86-mswin32-60 support

Per @luislavena on https://github.com/tjschuck/rake-compiler-dev-box/commit/ebec11e252b2adb8a932a02f7944f7c8d41825de, this was only necessary during the transition time from One-Click Ruby Installer (OCI) to RubyInstaller when 1.8.6 was released _7 years_ ago.