acpi: Tasks: Remove the now-redundant 'acpi_task_count'

[olce]

Summary

That number is used only up to the moment when the ACPI taskqueue's
threads are started to print the number of tasks queued so far. This is
now redundant with 'acpi_boot_tasks_nb'. So use the latter and remove
it, with the benefit that no more useless updates are performed at task
creation and end, which could create contention.