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/minimal/main.c

@@ -81,11 +81,6 @@ mp_import_stat_t mp_import_stat(const char *path) {
     return MP_IMPORT_STAT_NO_EXIST;
 }
 
-mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs) {
-    return mp_const_none;
-}
-MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
-
 void nlr_jump_fail(void *val) {
     while (1) {
         ;

ports/minimal/mpconfigport.h

@@ -25,10 +25,6 @@ typedef intptr_t mp_int_t; // must be pointer size
 typedef uintptr_t mp_uint_t; // must be pointer size
 typedef long mp_off_t;
 
-// 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) },
-
 // We need to provide a declaration/definition of alloca()
 #include <alloca.h>