hw/ppc/prep: Remove ifdeffed-out stub of XCSR code

hw/ppc/prep: Remove ifdeffed-out stub of XCSR code

The prep machine has some code which is stubs of accessors
for XCSR registers. This has been disabled via #if 0
since commit b6b8bd1819ff in 2004, and doesn't have any
actual interesting content. It also uses the deprecated
old_mmio accessor functions. Remove it entirely.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>