target-arm: Remove c0_cachetype CPUARMState field

Remove the no-longer-used CPUARMState c0_cachetype field.
Although this was a constant register we had it in our
migration state. Drop this (with resulting version bump)
because for ARM currently we prefer cleaner migration
code and have not stabilised migration format yet.

Signed-off-by: Peter Maydell <[email protected]>