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.

include/config/keybind.h

@@ -30,6 +30,8 @@ struct keybind {
  */
 struct keybind *keybind_create(const char *keybind);
 
+void keybind_destroy(struct keybind *keybind);
+
 /**
  * parse_modifier - parse a string containing a single modifier name (e.g. "S")
  * into the represented modifier value. returns 0 for invalid modifier names.