[tests] Remove maxblocksinflight testcase

Summary:
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

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, matiu

Reviewed By: #bitcoin_abc, matiu

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D991