Important: This answer is about the case when a hue is used that appears as a legend title. In all other cases, the question itself already contains the usual way to get rid of a title.
Indeed, seaborn is misusing a legend label as a (subgroup-)title. Hence the idea can be to either remove this label, or replace it with custom text.
Replacing with custom text:
legend = ax.legend()
legend.texts[0].set_text("Whatever else")
Removing the label:
handles, labels = ax.get_legend_handles_labels()
ax.legend(handles=handles[1:], labels=labels[1:])
After having removed the label you may of course still set another (real) title:
handles, labels = ax.get_legend_handles_labels()
ax.legend(handles=handles[1:], labels=labels[1:], title="Whatever else")
