ports: Remove typedef of machine_ptr_t, it's no longer needed.

This type was used only for the typedef of mp_obj_t, which is now defined
by the object representation.  So we can now remove this unused typedef,
to simplify the mpconfigport.h file.

examples/embedding/mpconfigport_minimal.h

@@ -127,9 +127,6 @@ typedef long long mp_off_t;
 typedef long mp_off_t;
 #endif
 
-typedef void *machine_ptr_t; // must be of pointer size
-typedef const void *machine_const_ptr_t; // must be of pointer size
-
 // We need to provide a declaration/definition of alloca()
 #ifdef __FreeBSD__
 #include <stdlib.h>