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/SIL/SILBasicBlock.h

@@ -54,7 +54,7 @@ private:
   SILBasicBlock() : Parent(nullptr) {}
   void operator=(const SILBasicBlock &) = delete;
 
-  void operator delete(void *Ptr, size_t) SWIFT_DELETE_OPERATOR_DELETED
+  void operator delete(void *Ptr, size_t) = delete;
 
   SILBasicBlock(SILFunction *F, SILBasicBlock *relativeToBB, bool after);