powerpc/mm: Remove useless #ifndef CPU_FTR_COHERENT_ICACHE in mem.c

Since commit 10b35d99 ("[PATCH] powerpc: merged asm/cputable.h"),
CPU_FTR_COHERENT_ICACHE has always been defined.

Remove the #ifndef CPU_FTR_COHERENT_ICACHE block.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e26ddc1d6f6aca739dd8d2b7c67351ead559b084.1602489664.git.christophe.leroy@csgroup.eu