do not sent Last-Modified if we expect a changed file

In 8d041b4f we made apt figure out based on the last Release file it has
if it should request a file or not given that the hashes changed or not.
So if we have a last Release file and do a request, do not sent a
Last-Modified header as we expect a change so much that a non-change
would indeed be an error. The Last-Modified header is therefore at best
ignored by the server, so sending it is just wasted effort. In the worst
case as time is a fragile thing the server decides against sending us an
update with the idea that we already have the latest content, which we
know for a fact that we haven't. Given that we sent less information to
the server our request is on its own also less identifiable as coming
from a returning or new user.

The disadvantage is that if we end up getting an old index file after
getting a new Release file from another mirror the old mirror will not
be able to tell us 'Hit', but instead sends us the complete file we
discard, but both lets us end up with the same error class in the end,
so the difference isn't big in practice.

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