[uaccess] Remove redundant virt_to_user() and userptr_t

Remove the last remaining traces of the concept of a user pointer,
leaving iPXE with a simpler and cleaner memory model that implicitly
assumes that all memory locations can be reached through pointer
dereferences.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/uaccess.c

@@ -34,4 +34,3 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 /* Flat address space user access API */
 PROVIDE_UACCESS_INLINE ( flat, phys_to_virt );
 PROVIDE_UACCESS_INLINE ( flat, virt_to_phys );
-PROVIDE_UACCESS_INLINE ( flat, virt_to_user );