s390/dasd: Remove unused structs and function prototypes

There are structs that have never been used. There are also two function
prototypes which were forgotton in commit f9f8d02f ("[S390] dasd:
revert LCU optimization").

Clean up and keep the header file tidy.

Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>