cli: fix warnings with -Wshadow

mmcli-manager.c: In function ‘context_free’:
  mmcli-manager.c:167:24: error: declaration of ‘ctx’ shadows a global declaration [-Werror=shadow]
    167 | context_free (Context *ctx)
        |               ~~~~~~~~~^~~
  mmcli-manager.c:51:17: note: shadowed declaration is here
     51 | static Context *ctx;
        |                 ^~~
  ...

cli/mmcli-modem-simple.c

@@ -101,7 +101,7 @@ mmcli_modem_simple_options_enabled (void)
 }
 
 static void
-context_free (Context *ctx)
+context_free (void)
 {
     if (!ctx)
         return;
@@ -131,7 +131,7 @@ ensure_modem_simple (void)
 void
 mmcli_modem_simple_shutdown (void)
 {
-    context_free (ctx);
+    context_free ();
 }
 
 static void