Remove the code for computing filetype (extension)

[obulat]

Blocked by https://github.com/WordPress/openverse-catalog/issues/520

Problem

We are currently using code to compute extension (filetype) for some images that do not have it in the database. This code should become unnecessary when we normalize data in the database. This issue is blocked by https://github.com/WordPress/openverse-catalog/issues/511

Description

We should remove the code that computes the extension during ES indexing as it should be available in the filetype field for all media items in the database. https://github.com/WordPress/openverse-api/blob/2e85caf7aede8aaf9d77cd5cb050f50b860ee58e/ingestion_server/ingestion_server/elasticsearch_models.py#L135-L144