FIX: Return figure handle in plot_drop_log

I removed the return parameter `perc` since it wasn't being used anywhere.
If I'm not the only one using it, then +1 for proper deprecation: 1) have `return_fig=True` mean return *only* the figure, 2) have `False` (default) give a deprecation warning, 3) add `drop_log_stats()` method (which I think would only really be a list comprension, but that's fine). WDYT?