grandpa: missing equivocation reporting nits (#5953)

* primitives: move reporting key type to common key types

* session: remove useless methods on MembershipProof

* grandpa: remove std special-casing when checking signatures

* grandpa: add some more docs

* grandpa: use proper error types rather than strings

substrate/primitives/session/src/lib.rs

@@ -63,18 +63,6 @@ pub struct MembershipProof {
 	pub validator_count: ValidatorCount,
 }
 
-impl MembershipProof {
-	/// Returns a session this proof was generated for.
-	pub fn session(&self) -> SessionIndex {
-		self.session
-	}
-
-	/// Returns the validator count of the session this proof was generated for.
-	pub fn validator_count(&self) -> ValidatorCount {
-		self.validator_count
-	}
-}
-
 /// Generate the initial session keys with the given seeds, at the given block and store them in
 /// the client's keystore.
 #[cfg(feature = "std")]