az login on headless vm opens w3m due to a regression.

**Describe the bug**
In a headless linux VM, doing `az login` opens www-browser and that in turn opens w3m, a text-only browser that does not support the device login flow.

**To Reproduce**
ssh into headless VM without X forwarding. run `az login`.

**Expected behavior**
Device code flow should get initiated, ie, i should see a URL and a code to enter there after opening it in a javascript-capable browser.

**Environment summary**
I used the script install on linux to get the newest version.

**Additional context**
This is a regression, which was introduced by [this commit](https://github.com/Azure/azure-cli/commit/1b4c85187cf0a74e4192e25f514f3c27edeef967). Ie, it doesn't check DISPLAY envvar anymore.