SUNRPC: Remove unused make_checksum

Commit ec596aaf ("SUNRPC: Remove code behind
CONFIG_RPCSEC_GSS_KRB5_SIMPLIFIED") was the last user of the
make_checksum() function.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>