Map folder within the client folder. It's generated only if you have "save map tiles to disk" option toggled on. Generally it's usable only if you want to export maps to external tools or are using algeralith's map uploader. It's not used by the client for displaying the map so you can just delete it and disable that option if you don't need it.
Map tiles used by the client are located under %APPDATA%\Roaming\Haven and Hearth. That one you probably don't want to delete since it also contains markers.
EDIT:
_Gunnar wrote:2) Could we get some more zoomed out map viewing levels? With the nomad quests I've crossed the world several times, and it nowhere near fits
Ah, I've remembered why I didn't add more levels initially. It made some nasty bug to resurface which I don't really want to start fixing.