REST API: Remove the `karma` property and query parameter from the Comments endpoints.

WordPress has not used the `karma` property internally for the past 8 years. There is no need to expose it in the REST API endpoints. Sites that use `karma` can include it using the `register_rest_field()` function.

Props dd32, danielbachhuber.
Fixes #38821.
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git-svn-id: https://core.svn.wordpress.org/trunk@39232 1a063a9b-81f0-0310-95a4-ce76da25c4cd