Role/Capability: Don't assign the `delete_site` capability to anyone on single site installs.

This capability is not relevant on single site installs; it only applies to deleting a site on a multisite installation.

Fixes #38326

Built from https://develop.svn.wordpress.org/trunk@39494


git-svn-id: https://core.svn.wordpress.org/trunk@39434 1a063a9b-81f0-0310-95a4-ce76da25c4cd