Remove avb_salt from filesystem and boot_img

avb_salt was added to filesystem and boot_img to create reproducible
builds https://r.android.com/2069528. This is no longer necessary, since
build_image.py and verity_utils.py has been updated since then to
prevent non-hermiticity from unspecified avb_salt.
https://r.android.com/2613794. It uses the build_fingerprint as the salt
instead of the filesystem entries.

In preparation for creating bit-identical make and soong partition
images, make the avb_salt consistent between make and soong partition
iamges.

Bug: 376539388
Test: lunch aosp_panther-trunk_staging-userdebug
Test: m
out/soong/.intermediates/packages/modules/Virtualization/guest/pvmfw/pvmfw_img/android_arm64_armv8-2a_cortex-a55/pvmfw.img
(a bootimage module) followed by rm repeatedly
Test: sha remains consistent

Change-Id: Ie1afa4599ef1de4c8f1cd708a52cb60b37c19373