Remove unused etag functionality

Checking the registry against a previous etag is now handled inside
flatpak_cache_http_uri(), so remove the etag parameters that were
previously passed around in various places for simplicity.

Closes: #1910
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f