platform/x86: x86-android-tablets: Simplify node-group [un]registration

platform/x86: x86-android-tablets: Simplify node-group [un]registration

software_node_register_node_group() / software_node_unregister_node_group()
both accept a NULL node-group as argument.

So there is no need to check for the node-group being NULL before calling
these functions, remove the checks to simplify the code.

Note the "if (gpio_button_swnodes)" check for registering is kept because
that also guards the creation of a gpio-button platform-device.

Suggested-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Hans de Goede <hansg@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250920200713.20193-15-hansg@kernel.org
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>