ACPI: driver: Do not set acpi_device_class() unnecessarily

ACPI: driver: Do not set acpi_device_class() unnecessarily

Several core ACPI device drivers set acpi_device_class() for the given
struct acpi_device to whatever they like, but that value is never used
unless the driver itself uses it and, sadly, they neglect to clear it on
remove.  Since the only one of them still using acpi_device_class()
after previous changes is the button driver, update the others to stop
setting it in vain.  Also drop the related device class sybmols that
become redundant.

Since the ACPI button driver continues to use acpi_device_class(), make
it clear the struct field represented by acpi_device_class() in its
remove callback.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/3706295.iIbC2pHGDl@rafael.j.wysocki