Revert "hw/i386/pc: Confine system flash handling to pc_sysfw"

Revert "hw/i386/pc: Confine system flash handling to pc_sysfw"

Specifying the property `-M pflash0` results in a regression:
  qemu-system-x86_64: Property 'pc-q35-9.0-machine.pflash0' not found
Revert the change for now until a solution is found.

This reverts commit 6f6ad2b24582593d8feb00434ce2396840666227.

Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20240226215909.30884-3-shentey@gmail.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>