Remove node-fetch dependency for unfetch TS definition file

[1999]

After a chat in #96 I decided to remove the node-fetch imports from unfetch definition file as now it produces TS compiler errors.

isomorphic-unfetch definion stayed untouched.

[AnselmMarie]

Is there an update on this PR?

[developit]

@1999 I'm so sorry for never responding to your PR. If possible, do you think you could take a look at #117 and tell me if that would work with your changes here?

[1999]

Yep, these changes look not related. Should be fine.

[developit]

I still need to check if this is correct. We landed #117 but I don't have a TypeScript project to test things out in.