Remove futility prebuilts.

The use of prebuilt futility tool has been removed in commit
e98fb7a8d3a460891f6c21fafbe62a86b50d3d1d (in platform/build project),
by switching to building from its source code instead (futility-host
module under external/vboot_reference/).

Bug: 35467608
Test: N/A
Change-Id: Ib7dd80ee463dadfbbada399c738b77a23503f4dc