remove marker tests

[jgravois]

i can/should get rid of the marker tests here, right?
https://github.com/Esri/esri-leaflet-renderers/tree/master/spec/Markers

[kneemer]

@jgravois, I had hoped to add a test that verified that the shapeMarkers library was loaded/working and then remove these tests but didn't get around to it.

[jgravois]

i can do that if you can sanity check the PR below 😉

Esri/Leaflet.shapeMarkers#22

i also understand if you ever decide you need to get away from both of these repos entirely.

[kneemer]

I put these repos into the wild so I feel some sense of stewardship still towards them 😄