libsnapshot: Remove various unused components.

PowerTest is no longer relevant with snapuserd, and was not particularly
good at rooting out dm issues anyway.

make_cow_from_ab_ota and estimate_cow_from_nonab_ota are no longer
relevant as they were mainly for estimation during VABC prototyping.

The update_metadata proto was part of the fuzzer which has since been
removed.

Bug: N/A
Test: th builds
Change-Id: Ie341e360f1824ea16794c4625817cfc8f8b8644b