Remove the spuriously failing test "dont_make_announcements_while_blocks_are_being_sent"

[ImplOfAnImpl]

Here I remove the spuriously failing test dont_make_announcements_while_blocks_are_being_sent, as well as the changes that were made when trying to fix it.
Two reasons for it:

1. The test checks a very minor improvement - the fact that the node should avoid sending header updates to a peer, while the peer is downloading blocks from it.
2. It turned out to be hard to avoid spurious failures in it - the test block that is supposed not to trigger an update may miss the window where the older blocks are being sent, so the update would actually be triggered.
   The only alternative to removing the test is to increase the number of initial blocks, so that they take longer time to send. But it slows down the test noticeably and still doesn't guarantee the absence of spurious failures.