libyang REFACTOR prefix format always decides the callback (#1169)

So the callback is no longer needed as a parameter.
Also, union no longer uses its own prefix mappings
but instead uses the original format and copies any
required prefix data.
diff --git a/src/tree_schema.c b/src/tree_schema.c
index fc73acf..fa9a8ee 100644
--- a/src/tree_schema.c
+++ b/src/tree_schema.c
@@ -309,7 +309,7 @@
     LY_CHECK_ERR_GOTO(!exp, ret = LY_EINVAL, cleanup);
 
     /* atomize expression */
-    ret = lyxp_atomize(exp, LYD_JSON, ctx_node->module, ctx_node, LYXP_NODE_ELEM, &xp_set, options);
+    ret = lyxp_atomize(exp, LY_PREF_JSON, ctx_node->module, ctx_node, LYXP_NODE_ELEM, &xp_set, options);
     LY_CHECK_GOTO(ret, cleanup);
 
     /* allocate return set */
@@ -353,7 +353,7 @@
     LY_CHECK_ERR_GOTO(!exp, ret = LY_EINVAL, cleanup);
 
     /* atomize expression */
-    ret = lyxp_atomize(exp, LYD_JSON, ctx_node->module, ctx_node, LYXP_NODE_ELEM, &xp_set, options);
+    ret = lyxp_atomize(exp, LY_PREF_JSON, ctx_node->module, ctx_node, LYXP_NODE_ELEM, &xp_set, options);
     LY_CHECK_GOTO(ret, cleanup);
 
     /* allocate return set */