Yes it is safe to uninstall AOSP keyboard.

I kinda feel uncomfortable writing a one lined answer... So I'll explain things a bit more in detail.

Basically, you can remove any app from android system as far as its not used by critical system functions. I dont not know of any specific up to date list that contains all apps that can be removed. But consider it this way:

What can a keyboard app be required for? Can it be used by some other app? The answer is pretty simple. The keyboard app only inputs user data. So if you have alternative, its pretty safe to remove. Even if you dont put *any* keyboard app at all, your system will work just fine even then. It will just get a little trickier to input stuff.