卸载jdk9

安装jdk8之前，先卸载jdk9，备忘录一下吧

sudo rm -rf /Library/Java/JavaVirtualMachines/具体版本jdk
sudo rm -fr /Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin 
sudo rm -fr /Library/PreferencePanes/JavaControlPanel.prefPane 
sudo rm -fr ~/Library/Application\ Support/Java