Remove split_six utility function

[Conengmo]

This utility function is not used anywhere in Branca or Folium. It sort of requires Pandas, which is not actually a dependency of Branca. By removing this function, we can also remove that awkward import of Pandas.

• This function was included in the first version, but was not used at that time as well.
• It's also not included in the documentation.
• I couldn't find a reference to it using Github's search function.

I think we're pretty safe to remove it, since I don't expect it will have downstream users. If there are downstream users, they can easily adopt the code themselves, since it's just a few lines.

[martinfleis]

Fine by me.

[Conengmo]

Thanks Martin for the review!