Fix system-as-root detection on Pi

From Pi, the manifest.xml had been moved from system folder,
so remove this file from system-as-root detection.

Bug: 73799919
Test: gsi_util dump --system some-aosp_arm64_ab-on-pi.img
Change-Id: I77e024d1200c46eb3bcdf6a853e1e02ae1493f52
1 file changed