refactor: remove timer utilities from Client

[kyranet]

Please describe the changes this PR makes and why it should be merged:

⚠️ This PR removes the following methods:

• BaseClient#setTimeout
• BaseClient#setInterval
• BaseClient#setImmediate
• BaseClient#clearTimeout
• BaseClient#clearInterval
• BaseClient#clearImmediate

Why?

The aforementioned methods existed to make timers not hang the process when it's destroyed and nothing else kept the process on (from the user's side). Since all timers have an unref method (Immediate#unref, Immediate#unref), there is not longer a need to keep and track those timers, which can potentially lead to memory leaks.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[SpaceEEC]

Is it really okay to not clear at least the intervals anymore?
A destroyed client will now continue sending typing start requests.
(As well as sweep messages and handlers, but those don't hit up Discord's api)

[kyranet]

A destroyed client will now continue sending typing start requests.

Fixed with #6114 😄

(As well as sweep messages and handlers, but those don't hit up Discord's api)

Assigned the relevant intervals and calling clear on them since the last commit, good catch 👍🏼

[iCrawl]

This needs a rebase.

[kyranet]

Done, @iCrawl