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-sms.c

@@ -123,7 +123,7 @@ mmcli_sms_options_enabled (void)
 }
 
 static void
-context_free (Context *ctx)
+context_free (void)
 {
     if (!ctx)
         return;
@@ -142,7 +142,7 @@ context_free (Context *ctx)
 void
 mmcli_sms_shutdown (void)
 {
-    context_free (ctx);
+    context_free ();
 }
 
 static void
@@ -411,7 +411,6 @@ mmcli_sms_run_synchronous (GDBusConnection *connection)
     if (store_in_storage_str) {
         gboolean operation_result;
         MMSmsStorage storage;
-        GError *error = NULL;
 
         storage = mm_common_get_sms_storage_from_string (store_in_storage_str, &error);
         if (error) {