[cxx-interop] Remove -fno-rtti workaround in tests

`-Xcc -fno-rtti` flags were added to these tests back in 2020 when we could not properly link with the C++ stdlib.

This should hopefully speed up these tests, which are currently among the slowest tests in the project.

Author: egorzhdan

test/Interop/Cxx/value-witness-table/custom-destructors-virtual-irgen.swift

@@ -1,8 +1,4 @@
-// With RTTI some of the objects with virtual bases / destructors in this test
-// will cause linker errors because of undefined vtables.
-// FIXME: Once we can link with libc++ we can start using RTTI.
-//
-// RUN: %target-swift-frontend -enable-experimental-cxx-interop -I %S/Inputs %s -emit-ir -Xcc -fno-rtti | %FileCheck %s
+// RUN: %target-swift-frontend -enable-experimental-cxx-interop -I %S/Inputs %s -emit-ir | %FileCheck %s
 
 import CustomDestructor
 

test/Interop/Cxx/value-witness-table/custom-destructors-virtual.swift

@@ -1,9 +1,5 @@
-// With RTTI some of the objects with virtual bases / destructors in this test
-// will cause linker errors because of undefined vtables.
-// FIXME: Once we can link with libc++ we can start using RTTI.
-// 
-// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -fno-rtti)
-// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -Xcc -fno-rtti -O)
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop)
+// RUN: %target-run-simple-swift(-I %S/Inputs -Xfrontend -enable-experimental-cxx-interop -O)
 //
 // REQUIRES: executable_test