[tests] Remove maxblocksinflight testcase

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.

test/functional/README.md

@@ -59,7 +59,7 @@ thread.)
 * RPC calls are available in p2p tests.
 
 * Can be used to write free-form tests, where specific p2p-protocol behavior
-is tested.  Examples: ```p2p-accept-block.py```, ```maxblocksinflight.py```.
+is tested.  Examples: ```p2p-accept-block.py```, ```p2p-compactblocks.py```.
 
 ## Comptool