kexec-tools: mips: Remove commandline parameter "mem"

"mem=" indicating the memory region the new kernel can use to boot into.
And passed to the dump-capture kernel by kernel commandline parameter
"mem=". But in the dump-capture kernel, we don’t need to use this parameter
now, so remove "mem" and don't add "mem=" to new kernel commandline.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>