hw/ppc/e500{, plat}: Drop redundant checks for presence of platform bus

This is a follow-up on commit 47a0b1df 'hw/ppc/mpc8544ds: Add
platform bus': Both mpc85xx boards now have a platform bus
unconditionally.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20230125130024.158721-3-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>