Fix gcc '-Wunused-but-set-variable' warning:

arch/powerpc/kernel/fadump.c: In function fadump_update_elfcore_header:
arch/powerpc/kernel/fadump.c:790:17: warning: variable elf set but not used [-Wunused-but-set-variable]

It is introduced by commit ebaeb5ae ("fadump:
Convert firmware-assisted cpu state dump data into elf notes."),
but never used, so remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1574144074-142032-2-git-send-email-zhengbin13@huawei.com