[FEA] Deprecate np.array fallback for ParallelPostFit predict function

[VibhuJawa]

We should np.array fallback when meta is None in ParallelPostFit predict in future releases to match dask's preferred behavior.

With PR #862 we add a meta argument to predict call and added a future warning about predict's meta fallback to np.array.

As per discussion here we should deprecate this fallback behavior in future releases.

With PR we added @pytest.mark.filterwarnings("ignore::FutureWarning") to some pytests to prevent pytest failure due to Future warning. (See the PR for details) . We should remove those when we remove the fallback.

Also remove the warnings ignored in documentation at commint 6c29844

CC: @TomAugspurger , @quasiben , @jakirkham

[VibhuJawa]

Closing issue cause we removed this behavior in #862 .

See comment for details.