ACPI / dock: Do not check CONFIG_ACPI_DOCK_MODULE

Since commit 94add0f8 (ACPI / dock: Initialize ACPI dock subsystem
upfront) the ACPI dock driver cannot be a module, so
CONFIG_ACPI_DOCK_MODULE is never set.  For this reason, simplify
the preprocessor conditional in include/acpi/acpi_drivers.h
referring to that sybbol unnecessarily.

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