Remove `Algebra.Definitions.Core(1/2)`

[MatthewDaggitt]

The module parameter setup just doesn't work. When you write

open import Function.Definitions

you still get e.g. Surjective in scope with two equality parameters instead of one.

I think the best plan is to ditch the module parameters in Function.Definitions entirely, and fold Core1 and Core2 into it.

[jamesmckinna]

How is this affected by @alexarice 's PR #1156 for a 'better' notion of Surjective, which IIUC, does use the two equality relations?

[MatthewDaggitt]

Ack, you're right. That would also fix it. Okay, lets merge that instead!