do not define STBI_ONLY_PNG in fuzz target

tests/stb_png_read_fuzzer.c

@@ -1,5 +1,5 @@
 #define STB_IMAGE_IMPLEMENTATION
-#define STBI_ONLY_PNG
+
 #include "../stb_image.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)