arm: defconfig: drop unused POWER_AVS option

Commit 785b5bb4 ("PM: AVS: Drop the avs directory and the
corresponding Kconfig") moved AVS code to SOC-specific folders, and
removed corresponding Kconfig from drivers/power, leaving original
POWER_AVS config option enabled in some defconfigs.

Remove the options, which have no references in the tree anymore.

Fixes: 785b5bb4

 ("PM: AVS: Drop the avs directory and the corresponding Kconfig")

Signed-off-by: Ross Burton <ross.burton@arm.com>