powerpc: Remove flush_instruction_cache() on 8xx

flush_instruction_cache() is never used on 8xx, remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/245cabd8f291facac8c8c5fd370e361a69e02860.1597384145.git.christophe.leroy@csgroup.eu