Abilities API: Revert Allow nested namespace ability names.

Abilities should follow the same pattern as all other namespaces and use a single namespace.

Reverts [61602].

See #64596.
Props jorgefilipecosta, justlevine, audrasjb.


git-svn-id: https://develop.svn.wordpress.org/trunk@62094 602fd350-edb4-49c9-b593-d223f7449a82