maxblocksinflight tested that a node would not send get_data messages
for more than 16 new blocks at the same time. bitcoin core no longer
responds to block invs with get_data, since it does headers-first
sync'ing. This test was therefore testing nothing and can be removed.
the sendheaders test script tests that bitcoin will not send get_headers
for more than 16 blocks simultaneously.
Backport core's PR10023