Revert "wait for 2 seconds on disconnect" changes

[halfalicious]

Changes were causing frequent exceptions during syncing (see #5835, #5828) due to Host::m_sessions and EthereumPeer::m_peers getting out-of-sync (if a peer reconnects during a pending disconnect), so we are reverting the changes in the 1.7 release until we can properly address the sync issues.

[codecov-io]

Codecov Report

Merging #5841 into release/1.7 will increase coverage by 0.01%.
The diff coverage is 75%.

@@               Coverage Diff               @@
##           release/1.7    #5841      +/-   ##
===============================================
+ Coverage         63.8%   63.81%   +0.01%     
===============================================
  Files              358      358              
  Lines            30595    30598       +3     
  Branches          3406     3406              
===============================================
+ Hits             19520    19525       +5     
- Misses            9846     9847       +1     
+ Partials          1229     1226       -3

[gumb0]

I think we shouldn't modify 1.7.0 changelog, as it's already released, so I added an entry about revert to 1.7.2 instead