hw/i386/pc_piix.c: always initialise ISA IDE drives in pc_init_isa()

hw/i386/pc_piix.c: always initialise ISA IDE drives in pc_init_isa()

By definition an isapc machine must always use ISA IDE drives so ensure that they
are always enabled. At the same time also remove the surrounding CONFIG_IDE_ISA
define since it will be enabled via the ISAPC Kconfig.

Signed-off-by: Mark Cave-Ayland <mark.caveayland@nutanix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250828111057.468712-15-mark.caveayland@nutanix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>