Remove vbmeta_system from GSI targets

This partially reverts commit 1785816c
so that the hash tree at the end of GSI is properly signed by AVB.

Bug: 161503491
Test: 'm dist' and check that vbmeta_system.im is gone
Test: boot GSI on android10 CF
Test: 'avbtool info_image --image system.img' and check AVB signing
Change-Id: I9f488380c0c410fdbd02c3c0803b6dc06cf7faab