Clean up core API endpoints

[viktorix]

We have two core endpoints that seem to be doing nothing:

• https://api-v1.classicpress.net/core/stable-check/1.0/
• https://api-v1.classicpress.net/core/version-check/1.0/

They correspond to WordPress APIs:

• https://api.wordpress.org/core/stable-check/1.0/
• https://api.wordpress.org/core/version-check/1.7/

These look like static JSON lists. We should be able to set this up. There's no reason to have outdated endpoints. If we don't need them, maybe we should remove them.