[cxx-interop] Remove test that triggered UB

Assigning a value to an empty `std::optional` is undefined behavior in C++.

Author: egorzhdan

test/Interop/Cxx/stdlib/use-std-optional.swift

@@ -16,11 +16,8 @@ StdOptionalTestSuite.test("pointee") {
   let pointee = nonNilOpt.pointee
   expectEqual(123, pointee)
 
-#if !os(Linux) // crashes on Ubuntu 18.04 (rdar://113414160)
   var modifiedOpt = getNilOptional()
-  modifiedOpt.pointee = 777
-  expectEqual(777, modifiedOpt.pointee)
-#endif
+  expectNil(modifiedOpt.pointee)
 }
 
 StdOptionalTestSuite.test("std::optional => Swift.Optional") {