This removes the deprecated batch verification. This was actually never really activated.
Nevertheless, we need to keep the host functions around to support old runtimes which may import
these host functions. However, we do not give access to these functions anymore. This means that any new
runtime can not call them anymore. The host function implementations we keep will not do batch verification and will
instead fall back to the always existing option of directly verifying the passed signature.
`finish_batch_verification` will return the combined result of all the batch verify calls.

This removes the `TaskExecutorExt` which only existed to support the batch verification. So, any
code that used this extension can just remove the registration of them. It also removes
`SignatureBatching` that was used by `frame-executive` to control the batch verification.
However, there wasn't any `Verify` implementation that called the batch verification functions.