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

arch/powerpc/kernel/btext.c

@@ -46,7 +46,6 @@ unsigned long disp_BAT[2] __initdata = {0, 0};
 static unsigned char vga_font[cmapsz];
 
 int boot_text_mapped __force_data = 0;
-int force_printk_to_btext = 0;
 
 extern void rmci_on(void);
 extern void rmci_off(void);