Removing --flag 2 when building GSI vbmeta.img

With the support of enabling AVB on standalone partitions in libfs_avb,
devices can boot GSI with dm-verity. No need to disable AVB anymore.

Devices still can use the following command to disable AVB on
device-specific vbmeta.img if needed:

    `fastboot flash --disable-verification vbmeta vbmeta.img`

Bug: 130595457
Test: Tree Hugger
Change-Id: I067dcda15f14f04428e0b60ce1f49227d61e4349