Delete legacy fs-verity support

The "legacy" fs-verity was introduced in P in Pixel 3 kernel. During
fs-verity upstream to Linux, the API has changed. During Pixel 4 / Q
development, fs-verity was upstreamed to Linux kernel. By setting
ro.apk_verity.mode = 2, device vendors such as Pixel 4 can opt in to
enable the support. The feature has become mandatory for new devices
shipped with R.

Since Pixel 3 family is no longer supported, it's time to remove the
dead code.

Bug: 120629632
Test: m
Test: TH
Change-Id: I6dacd9bbd38b502ee510ff12970e76342d7b72dd
Merged-In: I6dacd9bbd38b502ee510ff12970e76342d7b72dd