Revert "[TypeChecker] TypeChecker::isSubtypeOf should recognize Sendable subtyping"

This reverts commit 359ea14.

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -1068,17 +1068,13 @@ bool TypeChecker::typesSatisfyConstraint(Type type1, Type type2,
   }
 
   if (auto solution = cs.solveSingle()) {
-    const auto &score = solution->getFixedScore();
     if (unwrappedIUO)
-      *unwrappedIUO = score.Data[SK_ForceUnchecked] > 0;
+      *unwrappedIUO = solution->getFixedScore().Data[SK_ForceUnchecked] > 0;
 
     // Make sure that Sendable vs. no-Sendable mismatches are
     // failures here to establish subtyping relationship
     // (unlike in the solver where they are warnings until Swift 6).
     if (kind == ConstraintKind::Subtype) {
-      if (score.Data[SK_MissingSynthesizableConformance] > 0)
-        return false;
-
       if (llvm::any_of(solution->Fixes, [](const auto *fix) {
             return fix->getKind() == FixKind::AddSendableAttribute;
           }))