Revert "Floss: Fix build post repo migration"

This reverts commit ddd429fe.

As it turns out, we will need to change //bt to //bt/system after all
because the ChromeOS ebuild can't easily handle the subdirectory change
otherwise.

Bug: 205591223
Test: Run `./build.py`
Tag: #floss
Change-Id: I620221cb589712dfc1f84c7ea87a175634a86250