init: remove unfinished fsverity signature support for APEX sepolicy

The APEX sepolicy feature has unfinished support for verifying the
sepolicy file using fsverity with a builtin signature.  However, this
was never finished and doesn't really make sense, since the
already-implemented scheme that uses a full-file hash combined with a
userspace signature check is better suited to the problem.  Therefore,
remove this unfinished code.

Bug: 290064770
Test: presubmit and booting Cuttlefish
Change-Id: I3403a3303bcea32c7340642b843cd1541fe1fd2f