Remove watch poll feature

[andrewvc]

What does this PR do?

Removes the watch_poll feature, which has been deprecated since 7.0 in version 8.0.0.

Fixes #27167

Why is it important?

It's not needed anymore, and has long been flagged as obsolete. Also, cleans up the code.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[elasticmachine]

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• Start Time: 2021-08-04T18:02:21.261+0000

• Duration: 63 min 22 sec

• Commit: d495f9b

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Test	Results
Failed	0
Passed	3348
Skipped	80
Total	3428

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Test stats 🧪

Test	Results
Failed	0
Passed	3348
Skipped	80
Total	3428

[vigneshshanmugam]

LGTM pending some lint fixes.

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b remove-poll-watch upstream/remove-poll-watch
git merge upstream/master
git push upstream remove-poll-watch