pull: drop fetching_sync_uri

pull: drop fetching_sync_uri

This made sense back when we used a main loop even when we needed to
fetch objects synchronously. Nowadays, we no longer actually update
progress before the FETCHING_OBJECTS phase, which is only for async
requests.

This allows us to get rid of fetch_uri_contents_membuf_sync() and to
generalize fetch_uri_contents_utf8_sync() so that it only requires a
fetcher. This will be needed later.

Closes: #469
Approved by: cgwalters