[S390] adapt text to cu3088-removal

cu3088 layer for lcs and ctcm has been removed. Thus the reference
to cu3088 in this text is to be removed as well.

Signed-off-by: Ursula Braun <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

Documentation/s390/driver-model.txt

···
 private data.
 
 To implement a ccwgroup driver, please refer to include/asm/ccwgroup.h. Keep in
-mind that most drivers will need to implement both a ccwgroup and a ccw driver
-(unless you have a meta ccw driver, like cu3088 for lcs and ctc).
+mind that most drivers will need to implement both a ccwgroup and a ccw
+driver.
 
 
 2. Channel paths