powerpc: Remove flush_instruction_cache for book3s/32

The only callers of flush_instruction_cache() are:

arch/powerpc/kernel/swsusp_booke.S:	bl flush_instruction_cache
arch/powerpc/mm/nohash/40x.c:	flush_instruction_cache();
arch/powerpc/mm/nohash/44x.c:	flush_instruction_cache();
arch/powerpc/mm/nohash/fsl_booke.c:	flush_instruction_cache();
arch/powerpc/platforms/44x/machine_check.c:			flush_instruction_cache();
arch/powerpc/platforms/44x/machine_check.c:		flush_instruction_cache();

This function is not used by book3s/32, drop 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/50098f49877cea0f46730a9df82dcabf84160e4b.1597384512.git.christophe.leroy@csgroup.eu