ref(browser): Remove XHR transport from default bundle

[timfish]

Problem Statement

fetch is supported in all browsers supported by the browser SDK.

Solution Brainstorm

Can this be removed from the default bundle (along with the fetch detection) and only offered as an optional transport?

[AbhiPrasad]

Although we transpile to es5, we don't include polyfills for IE11 (like fetch), so this change would effectively mean that we drop support for IE11.

Another decision we can take is to drop the fetch transport and only use the XHR one by default - @lforst brought this up.

[timfish]

Don't we already require a list of polyfills for IE support and could we not just add fetch to that list?

[AbhiPrasad]

If you're referencing the page in our docs, that's because prior to 5.7.0 we required polyfills for IE11 support. After 5.7.0 we refactor the SDK to no longer require polyfills, see: #2243.

[timfish]

Ah right, I think I got this idea from Katies comment here but now realise required polyfills is not a thing yet!

For IE11, we've already told people that they have to use one of our ES5 bundles, to which we could add a polyfill if necessary

[AbhiPrasad]

Going to backlog this - will eventually happen but will take a while.