target-ppc: Drop redundant flags assignments from CPU families

Previous code has #define POWERPC_INSNS2_<family> PPC_NONE in some
places for macrofied assignment to insns_flags2 field.

PPC_NONE is defined as zero though and QOM classes are zero-initialized,
so drop any pcc->insns_flags2 = PPC_NONE; assignments.

PPC_NONE itself is still in use in translate.c.

Suggested-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
