ksmbd: remove unused ksmbd_file_table_flush function

ksmbd_file_table_flush is a leftover from SMB1. This function is no longer
needed as SMB1 has been removed from ksmbd.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>