Remove GKI from gsi_$arch

After having gki_$arch, there is no longer need for building GKI
by target gsi_$arch. The BoardConfigGkiCommon.mk is also moved
to /device/generic/common. The patch removes all GKI related
settings and files from target gsi_$arch.

Bug: 212486689
Test: build dist of gsi_arm64-userdebug
Test: build dist of aosp_arm64-userdebug
Change-Id: I369f42ad1f6e3f519bb62c3f035e699df1c7468f