gdb: Use C++17's std::make_unique instead of gdb::make_unique

gdb::make_unique is a wrapper around std::make_unique when compiled with
C++17.  Now that C++17 is required, use std::make_unique directly in the
codebase, and remove gdb::make_unique.

Change-Id: I80b615e46e4b7c097f09d78e579a9bdce00254ab
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net

gdb/python/py-mi.c

@@ -290,7 +290,7 @@ gdbpy_execute_mi_command (PyObject *self, PyObject *args, PyObject *kw)
   try
     {
       scoped_restore save_uiout = make_scoped_restore (&current_uiout, &uiout);
-      auto parser = gdb::make_unique<mi_parse> (std::move (mi_command),
+      auto parser = std::make_unique<mi_parse> (std::move (mi_command),
 						std::move (arg_strings));
       mi_execute_command (parser.get ());
     }

gdb/python/py-varobj.c

@@ -170,5 +170,5 @@ py_varobj_get_iterator (struct varobj *var, PyObject *printer,
       error (_("Could not get children iterator"));
     }
 
-  return gdb::make_unique<py_varobj_iter> (var, std::move (iter), opts);
+  return std::make_unique<py_varobj_iter> (var, std::move (iter), opts);
 }