[Elastic Agent] Remove deprecated --kibana-url.

[blakerouse]

What does this PR do?

Removes the --kibana-url from the install and enroll commands. This was original kept in to not break everyone as we switched to Fleet Server. Now with complete transition to Fleet Server this should be removed because it is confusing as it can now never point to Kibana.

Why is it important?

So its not confusing and so that --url is the only way forward.

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.

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[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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• Build Cause: Pull request #25529 updated

• Start Time: 2021-05-04T13:02:33.799+0000

• Duration: 103 min 41 sec

• Commit: 1bdc10f

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Test	Results
Failed	0
Passed	6781
Skipped	23
Total	6804

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Test stats 🧪

Test	Results
Failed	0
Passed	6781
Skipped	23
Total	6804

[ruflin]

LGTM.

@ph @urso I suggest we reach out to all the teams to validate this flag is not used anymore and then also backport it to 7.13.

[ph]

@blake-kephart Let's merge, @mdelapenya and @mtojek I think we have already adjusted the CI to now use the above flag?

[mdelapenya]

Confirmed that the e2e tests are not using that flag in master, will check backports for 7.x and 7.13.x