loader: Remove TLS

loader: Remove TLS

The only purpose TLS was serving was as a channel for allocation callbacks to
be useable inside cJSON and dirent_on_windows. TLS didn't solve any specific
threading problem. Instead it served as an information channel. On top of that,
the only time when the TLS was used was inside vkCreateInstance as the
pre-instance calls would have the TLS pointer to loader_instance be set to
NULL.