lisa.stats: Remove constant column from plot_stats() labels
lisa.stats: Remove constant column from plot_stats() labels

When dataframes have a tag column that is constant inside a subgroup and
group, it means it is redudant (i.e. it does not help in identifying a value
uniquely), and can therefore be dropped from labels to gain in clarity.

credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa