[SIL] Remove what amounts to dead code from classifyDynamicCast().

The archetype tests in the code being removed will never succeed
because there is code earlier in the function testing for any
archetypes in either type, and that path always returns.

The remaining test is broken (it's comparing source input and target
input and then target input to target result rather than comparing the
source input and target input and then source result and target
result), and is just duplicating the test that happens above.

If we really want to do a better test for substitutability we can do
so at a later time. In the meantime, I don't think it makes sense to
leave redundant comparisons and an unreachable return in place.

lib/SIL/DynamicCasts.cpp

@@ -490,17 +490,6 @@ swift::classifyDynamicCast(ModuleDecl *M,
           && sourceFunction.getResult() == targetFunction.getResult())
         return DynamicCastFeasibility::WillSucceed;
 
-      auto isSubstitutable = [](CanType a, CanType b) -> bool {
-        // FIXME: Unnecessarily conservative; should structurally check for
-        // substitutability.
-        return a == b || a->hasArchetype() || b->hasArchetype();
-      };
-    
-      if (isSubstitutable(sourceFunction.getInput(), targetFunction.getInput())
-          && isSubstitutable(targetFunction.getInput(),
-                             targetFunction.getResult()))
-        return DynamicCastFeasibility::MaySucceed;
-      
       return DynamicCastFeasibility::WillFail;
     }
   }