misc: Replace the GEM5_VAR_USED macro with [[maybe_unused]].

The [[maybe_unused]] attribute is now standard, so we can use that
directly without hiding it behind a macro.

Change-Id: If24ffd7e50bdb503cb3e6ea61f226ea794e84b8f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48511
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/SimObject.py

@@ -889,7 +889,7 @@ module_init(py::module_ &m_internal)
                 # method, or the Dummy one. Either an implementation is
                 # mandantory since this was shunted off to the dummy class, or
                 # one is optional which will override this weak version.
-                code('GEM5_VAR_USED ${{cls.cxx_class}} *')
+                code('[[maybe_unused]] ${{cls.cxx_class}} *')
                 code('Dummy${cls}Shunt<${{cls.cxx_class}}>::Params::create() '
                      'const')
                 code('{')