kexec, remove panic_on_warn kernel parameter from kdump situations

panic_on_warn kernel parameter will cause the kernel to panic when a
WARN() is hit in the kernel.  This is not a good situation for the kdump
kernel because then it would be possible for the kdump kernel to panic in
a non-fatal WARN().

This patch removes panic_on_warn as a kernel parameter for the kdump
kernel.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: WANG Chao <chaowang@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>