Remove Node.js 6 support

[gengjiawen]

Since Node.js is not support anymore. It's maybe good to remove related code and test.

[rvagg]

PR would be good.

This would be good to do in 6, or 7 (not planned yet but might be something we do in time for Node 14), but we should probably be more careful with 5, for people who are stuck.

[richardlau]

If it's in a semver-major I'd be okay dropping Node.js 8 support too.

[codler]

Node js 8 is only supported untill 2019-12-31, so might be good to drop node 8 also