remove VS2015 workaround (NFC)

VS2015 had an issue with the deletion of an operator.  Since VS2017 is
the minimum version that LLVM uses, we can assume that VS2017+ is in use
(_MSC_VER >= 1910).  Clean up the now defunct workaround.

include/swift/AST/Module.h

@@ -582,7 +582,7 @@ public:
 private:
   // Make placement new and vanilla new/delete illegal for Modules.
   void *operator new(size_t Bytes) throw() = delete;
-  void operator delete(void *Data) throw() SWIFT_DELETE_OPERATOR_DELETED;
+  void operator delete(void *Data) throw() = delete;
   void *operator new(size_t Bytes, void *Mem) throw() = delete;
 public:
   // Only allow allocation of Modules using the allocator in ASTContext