s390/setup: remove unused symbolic constants for C code from setup.h

These symbolic constants are used only by assembler code now:
* COMMAND_LINE
* IPL_DEVICE

C code of the decompressed kernel should use boot data passed
by the decompressor instead.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>