[keyserver][lib] Get rid of unused code for handling videos in mediaFromRow

Summary: This code is actually only ever used for handling messageTypes.IMAGES, which doesn't have any video. We accidentally included it, but it's never used.

Test Plan:
To confirm that the fetch path would never hit videos, I ran this query on my production keyserver and got no results:

SELECT m.*, u.type AS upload_type FROM messages m LEFT JOIN uploads u ON u.container = m.id WHERE m.type = 14 AND u.type != 'photo' LIMIT 5;

I also consulted @atul, and we read through the code to make sure the upload path doesn't use messageTypes.IMAGES if there are any videos. Also Flow.

Reviewers: atul, bartek

Reviewed By: atul

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7336