Docs: Remove extra '--request GET' from cURL examples

What does this MR do?

Several cURL examples in the REST API documentation use --request GET.
This is unnecessary as cURL infers the GET request method.

When you run cURL in verbose mode it even informs you:

Note: Unnecessary use of -X or --request, GET is already inferred.

See also https://daniel.haxx.se/blog/2015/09/11/unnecessary-use-of-curl-x/

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
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