scsi: zfcp: drop old default switch case which might paper over missing case

This was introduced with v2.6.27 commit 287ac01a ("[SCSI] zfcp: Cleanup
code in zfcp_erp.c") but would now suppress helpful -Wswitch compiler
warnings when building with W=1 such as the following forced example:

drivers/s390/scsi/zfcp_erp.c: In function 'zfcp_erp_setup_act':
drivers/s390/scsi/zfcp_erp.c:220:2: warning: enumeration value 'ZFCP_ERP_ACTION_REOPEN_PORT' not handled in switch [-Wswitch]
  switch (need) {
  ^~~~~~

But then again, only with W=1 we would notice unhandled enum cases.
Without the default cases and a missed unhandled enum case, the code might
perform unforeseen things we might not want...

As of today, we never run through the removed default case, so removing it
is no functional change.  In the future, we never should run through a
default case but introduce the necessary specific case(s) to handle new
functionality.

Signed-off-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>