Remove `dec_weak_task_count` and `inc_weak_task_count` from `rust_kernel`

[brson]

register_task/unregister_task and dec_weak_task_count/inc_weak_task_count do the same thing.

[luqmana]

So, should register_task/unregister_task then be exposed in rust_builtin so it can be called from Rust?

[brson]

@luqmana Yes, though I suggest renaming it, since it will no longer just be used for 'registering', nor does it strictly have anything to do with tasks or weak tasks from the kernel's perspective - this count is only for determining when to shutdown the runtime. Maybe inc_live_count for the method name and rust_inc_kernel_live_count for the builtin. It's not great either, but is less wed to implementation details.