configs: mediatek: drop unused CONFIG_MTK_POWER_DOMAIN

configs: mediatek: drop unused CONFIG_MTK_POWER_DOMAIN

The MT7981, MT7986, MT7987 and MT7988 SoCs do not have a scpsys power
domain block: none of their device trees carry a "mediatek,*-scpsys"
compatible, so the driver never binds and enabling CONFIG_MTK_POWER_DOMAIN
only builds dead code.

Note: the MT7987 and MT7988 device trees do carry a
"mediatek,mt79xx-power-controller" compatible on their topmisc syscon,
but no U-Boot driver binds it and no node consumes power-domains, so
the symbols are unused there as well.

Remove the unused symbols (CONFIG_MTK_POWER_DOMAIN and
CONFIG_POWER_DOMAIN) from these defconfigs.

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Link: https://patch.msgid.link/20260709-mt8188-add-power-domain-v2-2-589ace7d30e2@baylibre.com
Signed-off-by: David Lechner <dlechner@baylibre.com>