Remove the Watermark API endpoint

[zackkrida]

Problem

The API currently contains a deprecated endpoint:

https://api.openverse.org/v1/#tag/images/operation/images_watermark

This endpoint only works for some images, and, for example, does not support SVGs.

Furthermore, the image is not accessible and low-quality. The attribution statement is hard-coded into image which has major legibility problems. The frame and the fonts are non-configurable and frankly look outdated.

Additionally, this endpoint sees little to no usage.

Description

Remove this endpoint entirely. This is complicated by our lack of clear API versioning and is blocked until we resolve that problem. #662 will address the problem.

Alternatives

Additional context