Remove stb_rect_pack implementation from stb_truetype (fix #4215)

vendor/stb/src/stb_truetype.c

@@ -1,5 +1,2 @@
-#define STB_RECT_PACK_IMPLEMENTATION
-#include "stb_rect_pack.h"
-
 #define STB_TRUETYPE_IMPLEMENTATION
 #include "stb_truetype.h"