move stage3_func typedef out of shared internal dynlink.h header

this interface contract is entirely internal to dynlink.c.

src/internal/dynlink.h

@@ -96,7 +96,6 @@ struct fdpic_dummy_loadmap {
 #define DYN_CNT 32
 
 typedef void (*stage2_func)(unsigned char *, size_t *);
-typedef void (*stage3_func)(size_t *);
 
 hidden void *__dlsym(void *restrict, const char *restrict, void *restrict);