Remove 'fleet-server-insecure-http' setting from docs

[kilfoyle]

Apparently the setting fleet-server-insecure-http was removed by elastic/kibana#129371
This updates the docs to remove mention of the removed setting.

Closes: #1273

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[kilfoyle]

@nchaulet When you have a minute can you review this one, please?

[nchaulet]

@kilfoyle I am not sure about that one, the UI do not allow to setup an http fleet server, but the option --fleet-server-insecure-http still exists in fleet server and it possible to create an http fleet server hosts with the Fleet API, this is only used for testing/development.

[kilfoyle]

Thanks @nchaulet! I'll update the parent issue accordingly.

[kilfoyle]

Actually, if the setting is used only for testing & development, perhaps we should just remove it from the docs anyway. What do you think?

[nchaulet]

Actually, if the setting is used only for testing & development, perhaps we should just remove it from the docs anyway. What do you think?

Yes it's probably better as there is no reason for an end user to use those settings