rust/hpet: Drop BqlCell wrapper for num_timers

rust/hpet: Drop BqlCell wrapper for num_timers

Now that the num_timers field is initialized as a property, someone may
change its default value using qdev_prop_set_uint8(), but the value is
fixed after the Rust code sees it first.  Since there is no need to modify
it after realize(), it is not to be necessary to have a BqlCell wrapper.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250520152750.2542612-4-zhao1.liu@intel.com
[Remove .into() as well. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>