Deprecate `--track-save` option from `bst build`

It makes no sense to support this now that we are adding project.refs,
instead of removing the option completely, we document it as
deprecated and print a warning.

  o _frontend/cli.py: Mark the option as deprecated, warn if it's used

  o _scheduler/trackqueue.py: Remove save optionality

  o _pipeline.py: Remove save optionality

  o tests/frontend/buildtrack.py: Stop testing for no-save functionality,

    This test was actually broken, and only save functionality was being
    tested. This was due to using a list [True] or [False] in the
    @pytest.mark.parameterize() statement, both of which are truthy values.