Rename tree_close() to tree_close_internal().

It should be clear for callers of this function that this is an internal
function that skips certain validations which might be important. Therefore
we make it clear that this is an internal function by renaming it.

relates to #1761

src/commands.c

@@ -1281,7 +1281,7 @@ void cmd_kill(I3_CMD, const char *kill_mode_str) {
     else {
         TAILQ_FOREACH(current, &owindows, owindows) {
             DLOG("matching: %p / %s\n", current->con, current->con->name);
-            tree_close(current->con, kill_mode, false, false);
+            tree_close_internal(current->con, kill_mode, false, false);
         }
     }