s390/mm: remove redundant check against VMEM_MAX_PHYS

The value of ident_map_size could never exceed the value of
vmemmap as secured by setup_kernel_memory_layout() function:

	/* make sure identity map doesn't overlay with vmemmap */
	ident_map_size = min(ident_map_size, vmemmap_start);

Since VMEM_MAX_PHYS macro is set to vmemmap and a newly added
range is checked against ident_map_size in add_memory_merged()
function anyway, the check against VMEM_MAX_PHYS is redundant.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>