gdb, testsuite, fortran: fix double free in mixed-lang-stack.exp

While testing mixed-lang-stack I realized that valgrind actually
complained about a double free in the test.

   All done
  ==2503051==
  ==2503051== HEAP SUMMARY:
  ==2503051==     in use at exit: 0 bytes in 0 blocks
  ==2503051==   total heap usage: 26 allocs, 27 frees, 87,343 bytes allocated
  ==2503051==
  ==2503051== All heap blocks were freed -- no leaks are possible
  ==2503051==
  ==2503051== For lists of detected and suppressed errors, rerun with: -s
  ==2503051== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

Reason for this is that in mixed-lang-stack.cpp in mixed_func_1f an
object "derived_type obj" goes on the stack which is then passed-by-value
(so copied) to mixed_func_1g.  The default copy-ctor will be called but,
since derived_type contains a heap allocated string and the copy
constructor is not implemented it will only be able to shallow copy the
object.  Right after each of the functions the object gets freed - on the
other hand the d'tor of derived_type actually is implemented and calls
free on the heap allocated string which leads to a double free.  Instead
of obeying the rule of 3/5 I just got rid of all that since it does not
serve the test.  The string is now just a const char* = ".." object
member.

gdb/testsuite/gdb.fortran/mixed-lang-stack.cpp

@@ -26,17 +26,7 @@ class base_one
 class base_two
 {
 public:
-  base_two ()
-  {
-    string = strdup ("Something in C++");
-  }
-
-  ~base_two ()
-  {
-    free (string);
-  }
-
-  char *string = nullptr;
+  const char *string = "Something in C++";
   float val = 3.5;
 };