Multisite: Delete the signup entry when a user is deleted from the database.

When a user has registered with incorrect data and got the account deleted immediately, it currently wasn't possible to register with the same login or email address again due to the existing signup entry. They had to wait for two days until the entry gets automatically deleted. Now the associated signup entry gets deleted as part of the account deletion.

Fixes #43232.
Built from https://develop.svn.wordpress.org/trunk@48315


git-svn-id: https://core.svn.wordpress.org/trunk@48084 1a063a9b-81f0-0310-95a4-ce76da25c4cd