Remove code that's supporting older platforms (in v2)

[lawrence-forooghian]

In particular, owenpearson said there is a bunch of stuff in utils.ts that can be replaced by built-in ES stuff. (Once we've done #1537 we'll know what version of ES we're targeting.)

There may also be other stuff that's supporting older browsers; again, waiting for outcome of #1537 to know what we can remove.

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3987

[VeskeR]

Based on DR accepted in #1537, the code to remove would include:

• IE specific code (can be found by searching for words IE, MSIE, IE[\d+], IE [\d+], Internet Explorer)
• functions/class methods polyfills that are natively supported by ES2017 (Array.filter, Array.map etc if we don't use it already. mostly found in utils.ts
• old browser versions specific code that we don't support anymore (old versions of Chrome, Firefox, Safari etc)

[VeskeR]

Done in #1629