docs: update image block extraction docs (#2578)
This PR removes `extract_image_block_to_payload` section from "API
Parameters" page. The "unstructured" API does not support the
`extract_image_block_to_payload` parameter, and it is always set to
`True` internally on the API side when trying to extract image blocks
via the API. Users only need to specify `extract_image_block_types`
parameter when extracting image blocks via the API.
**NOTE:** The `extract_image_block_to_payload` parameter is only used
when calling `partition()`, `partition_pdf()`, and `partition_image()`
functions directly.
### Testing
CI should pass.
