zephyr/mpconfigport: Remove duplicate builtins.open definition.

It's already provided because `MICROPY_PY_IO` is enabled.

Tested on `bbc_microbit_v2`, there is now only a single open.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

ports/zephyr/mpconfigport.h

@@ -181,10 +181,6 @@ typedef long mp_off_t;
 
 #define MP_SSIZE_MAX (0x7fffffff)
 
-// extra built in names to add to the global namespace
-#define MICROPY_PORT_BUILTINS \
-    { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) },
-
 #if MICROPY_PY_THREAD
 #define MICROPY_EVENT_POLL_HOOK \
     do { \