Delete unused API for legacy fs-verity

The "legacy fs-verity" (that was supported on Pixel 3 family) has been
removed in ag/16713104. As there is no more client, the legacy methods
in installd can now be deleted.

Bug: 120629632
Test: m
Change-Id: I56aa87d0922b0666f80d883d36f0900ffa069a31
Merged-In: I56aa87d0922b0666f80d883d36f0900ffa069a31