rcxml.c: fix mem leak when deduplicating keybinds

Before this patch `keybind->keysyms` wasn't free'd when
- deduplicating keybinds
- removing keybinds due to empty action list

This patch creates a shared `keybind_destroy()` helper
which gets used in all cases where a keybind is destroyed.

src/config/keybind.c

@@ -188,3 +188,12 @@ keybind_create(const char *keybind)
 	wl_list_init(&k->actions);
 	return k;
 }
+
+void
+keybind_destroy(struct keybind *keybind)
+{
+	assert(wl_list_empty(&keybind->actions));
+
+	zfree(keybind->keysyms);
+	zfree(keybind);
+}