s390/setup: Drop stale ident_map_size declaration
ident_map_size is no longer a standalone variable and the declaration
in asm/setup.h conflicts with its current definition in asm/page.h
introduced with commit 236f324b7473 ("s390/mm: Create virtual memory
layout structure"). Remove the stale declaration.
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>