use internal fetch as replacement for node-fetch (#3184)

related to #2649

I was able to move to internal fetch and all tests seems fine so far.

But we have one problem with the calendar module. In the docs we have
several authentication methods and one of them is `digest`. For this we
used `digest-fetch` which needs `node-fetch` (this is not so clear from
code but I was not able to get it working).

So we have 3 options:
- remove `digest` as authentication method for calendar module (this is
what this PR does at the moment)
- find an alternative npm package or implement the digest stuff
ourselves
- use `digest-fetch` and `node-fetch` for calendar module (so they would
remain as dependencies in `package.json`)

Opinions? @KristjanESPERANTO @rejas @sdetweil @MichMich

js/server_functions.js

@@ -1,7 +1,6 @@
 const fs = require("fs");
 const path = require("path");
 const Log = require("logger");
-const fetch = require("./fetch");
 
 /**
  * Gets the config.