nuttx: Remove fdopendir (#1895)

The newer version of nuttx has implemented fdopendir, and the compiler
reports multiple definitions of it.

core/shared/platform/nuttx/nuttx_platform.c

@@ -144,10 +144,3 @@ utimensat(int fd, const char *path, const struct timespec ts[2], int flag)
 }
 
 #endif /* !defined(AT_FDCWD) */
-
-DIR *
-fdopendir(int fd)
-{
-    errno = ENOSYS;
-    return NULL;
-}