Plugins: Make delete plugin message less scary.

This adds a check if the plugin actually has an uninstall routine before saying that its data will be deleted too.

Props samful, joostdevalk, joyously, Chouby, SergeyBiryukov.
Fixes #50346.
Built from https://develop.svn.wordpress.org/trunk@48451


git-svn-id: https://core.svn.wordpress.org/trunk@48220 1a063a9b-81f0-0310-95a4-ce76da25c4cd