[net processing] Remove dropmessagestest

Summary:
-dropmessagestest is a command line option that causes 1 in n received
messages to be dropped. The Bitcoin P2P protocol is stateful and in
general cannot handle messages being dropped. Dropped
version/verack/ping/pong messages will cause the connection to time out
and be torn down. Other dropped messages may also cause the peer to
believe that the peer has stalled and tear down the connection.

It seems difficult to uncover any actual issues with -dropmessagestest,
and any coverage that could be generated would probably be easier to
trigger with fuzz testing.

This is a backport of core#20747

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10948