Stage 2 of refactoring of Relation.Nullary

[MatthewDaggitt]

Deprecates and cleans up the imports

Relation.Nullary.Product
Relation.Nullary.Sum
Relation.Nullary.Implication

[jamesmckinna]

Asleep at the wheel, so I missed these:

• Relation.Nullary.Negation.Core has two redundant imports

• open import Data.Bool.Base using (not)
• open import Data.Product.Base using (_×_; _,_; proj₁; proj₂)

Suggest a second round of import-bashing through this PR and its predecessor #1855?
UPDATED Additionally, can tighten those for

• Relation/Nullary/Decidable.agda
• Relation/Nullary/Negation.agda
• Relation/Nullary/Reflects.agda

[MatthewDaggitt]

Sorry, only just seen this. Yes, would welcome these import changes 👍