powerpc: remove set but not used variable 'force_printk_to_btext'

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

arch/powerpc/kernel/btext.c:49:12: error: 'force_printk_to_btext'
defined but not used.

It is never used, and so can be removed.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210408011801.557004-2-yukuai3@huawei.com