[Dexter] Remove clang-opt-bisect analyzer tool (#128165)

The idea behind this tool was that you'd instrument a source file to
measure its debug-info quality, then watch as it progressively got worse
with more optimisations being enabled in clang. However we've since
stripped the "building" portions of Dexter out as they were ill placed,
which makes this tooling redundant. The lack of __init__.py adjacent to
it means it couldn't be run anyway.

The core idea behind this is still sound; just it's best placed on the
other side of the build system, something that Dexter shouldn't try to
solve.

Author: jmorse