fetcher: Drop max queue size assertion in libsoup/libcurl backends

fetcher: Drop max queue size assertion in libsoup/libcurl backends

Since f4d1334e19ce3ab2f8872b1e28da52044f559401 the primary pull code maintains a
maximum queue. In that commit message I said `Note that I kept an assertion.`.
But I think this is wrong since while it covers a lot of the normal cases, if
one is e.g. trying to fetch a ton of refs, the primary pull code doesn't yet
queue those.  While it'd be nice to queue those, it isn't worth carrying
extra assertions in the backends that can still trigger.

Closes: https://github.com/ostreedev/ostree/issues/1451
Closes: #1453
Approved by: dbnicholson