Metrowerks: remove duplicate optimization args

The args were in both buildtype and optimization. This broke buildtypes other
than plain or custom unless manually setting the optimization level to
0, because Metrowerks chokes on duplicate arguments.

mesonbuild/compilers/mixins/metrowerks.py

@@ -32,10 +32,10 @@ else:
 
 mwcc_buildtype_args: T.Dict[str, T.List[str]] = {
     'plain': [],
-    'debug': ['-g'],
-    'debugoptimized': ['-g', '-O4'],
-    'release': ['-O4,p'],
-    'minsize': ['-Os'],
+    'debug': [],
+    'debugoptimized': [],
+    'release': [],
+    'minsize': [],
     'custom': [],
 }