remove convert methods

[rafaqz]

I went to start rewriting convert for GeoInterface but it just doesn't make sense to convert to these objects. We can write any geointerface geometries to shapefile anyway now, and it's a useless format for intermediate use. So its better to not maintain this code.

So this PR just removes all the convert methods.

[visr]

I'm fine with this. I hope this code is hardly used. It is still breaking though. Does it make sense to bring Shapefile.jl to 1.0? We could have a look if there are other possible things that we still want to change.

[rafaqz]

I doubt anyone has ever used it... we only got write recently so there was no reason to convert, and you don't need to convert to write anyway! It was just a dumb thing I wrote trying to implement all of GeoInterface here.

But yes a 1.0 could be nice, although we could probably fix a few things before then?

I never loved the name of Shapefile.Handle...

[visr]

Ok then perhaps we can just merge this as non breaking.

Indeed I'm not a fan of Handle either.