[Sema] drop half-baked redundancy checking for now

We already don't diagnose all redundant requirements. Because inverses
can be written in both inheritance and where clauses, but they're not
treated uniformly in the implementation, it's a bit annoying to try and
account for the redundancies in both places; `checkInheritanceClause`
will go over the same "requirements" that we'll also check again in
`swift::rewriting::expandDefaultRequirements`.

test/Generics/inverse_protocols_errors.swift

@@ -41,9 +41,3 @@ struct NCThinger<T: ~Copyable>: ~Copyable, Hello {
   // expected-note@-3 {{add 'inout' for a mutable reference}}
   // expected-note@-4 {{add 'consuming' to take the value from the caller}}
 }
-
-struct ExtraNoncopyStruct: ~Copyable, ~Copyable {}
-// expected-error@-1 {{duplicate inverse constraint}}
-// expected-note@-2 {{previous inverse constraint here}}
-
-protocol ExtraNoncopyProto: ~Copyable, ~Copyable {}