Role/Capability: Use meta caps `edit_post`, `read_post`, and `delete_post` directly.

Rather than consulting the post type object, let `map_meta_cap()` handle that for us.

Props peterwilsoncc, ocean90.
Fixes #50128.
See #23226.
Built from https://develop.svn.wordpress.org/trunk@47850


git-svn-id: https://core.svn.wordpress.org/trunk@47626 1a063a9b-81f0-0310-95a4-ce76da25c4cd