fs: cifs: mute -Wunused-const-variable message

After 'Initial git repository build' commit,
'mapping_table_ERRHRD' variable has not been used.

So 'mapping_table_ERRHRD' const variable could be removed
to mute below warning message:

   fs/cifs/netmisc.c:120:40: warning: unused variable 'mapping_table_ERRHRD' [-Wunused-const-variable]
   static const struct smb_to_posix_error mapping_table_ERRHRD[] = {
                                           ^
Signed-off-by: Austin Kim <austindh.kim@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>