ACPI / cpuidle: Drop flags.bm_check tests from acpi_idle_enter_bm()

Since acpi_idle_enter_bm() is only used if flags.bm_check is set for
the given acpi_processor object, it doesn't make sense to check that
flag in there.

For this reason, drop flags.bm_check tests (and some code depending
on them) from acpi_idle_enter_bm().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>