acpi_timer(4): Remove unused 'acpi_timer_disabled' boolean

Same reason as for the previous commit to acpi_cpu(4). This boolean is
not used anywhere. Disabling acpi_timer(4) can be done through the
regular ACPI disable mechanism (using the 'debug.acpi.disabled' tunable,
see acpi_disabled()).

Reviewed by: emaste (implicit)
Fixes: ac3ede5371af ("x86/xen: remove PVHv1 code")
MFC after: 3 days
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