[CIFS] Replace cifs md5 hashing functions with kernel crypto APIs

Replace remaining use of md5 hash functions local to cifs module
with kernel crypto APIs.
Remove header and source file containing those local functions.

Signed-off-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>