Can remove the enum_to_string dependency

[deandreamatias]

Can remove the dependency enum_to_string. This feature is embedded on Dart currently.

Change this places_
https://github.com/search?q=repo%3Amapbox%2Fmapbox-maps-flutter+enum_to_string&type=code

the dependency enum_to_string currently have a vulnerability:
https://nvd.nist.gov/vuln/detail/CVE-2017-16116

[maios]

Hi @deandreamatias, thank you for raising this. We have removed enum_to_string from our pubspec in #575