ports: Use default VFS config for import_stat and builtin_open.

For ports with MICROPY_VFS and MICROPY_PY_IO enabled their configuration
can now be simplified to use the defaults for mp_import_stat and
mp_builtin_open.

This commit makes no functional change, except for the following minor
points:
- the built-in "open" is removed from the minimal port (it previously did
  nothing)
- the duplicate built-in "input" is removed from the esp32 port
- qemu-arm now delegates to VFS import/open

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

ports/unix/mpconfigport.h

@@ -214,8 +214,6 @@ extern const struct _mp_print_t mp_stderr_print;
 #define MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE  (256)
 #define MICROPY_ASYNC_KBD_INTR      (1)
 
-#define mp_import_stat mp_vfs_import_stat
-#define mp_builtin_open_obj mp_vfs_open_obj
 #define mp_type_fileio mp_type_vfs_posix_fileio
 #define mp_type_textio mp_type_vfs_posix_textio
 
@@ -273,9 +271,6 @@ void mp_unix_mark_exec(void);
 #endif
 #endif
 
-#define MICROPY_PORT_BUILTINS \
-    { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) },
-
 #define MP_STATE_PORT MP_STATE_VM
 
 #if MICROPY_PY_BLUETOOTH