powerpc/book3s64/hash: Drop WARN_ON in hash__remove_section_mapping()

The single caller (arch_remove_linear_mapping()) prints a proper
warning when this function fails. No need to eventually crash the
kernel - let's drop this WARN_ON.

Suggested-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201111145322.15793-7-david@redhat.com