fix(tests): remove undeeded broken tests

[jansegre]

This test broke due to a regression after the latest on_new_tx refactor. However this test isn't actually adding anything right now. This should be reviewed in #648

Acceptance criteria

• test_checkpoint_validation should be removed
• all tests should now pass on the CI run

[codecov]

Codecov Report

Merging #668 (7ac143f) into master (b0e3509) will increase coverage by 0.00%.
The diff coverage is 81.63%.

@@           Coverage Diff           @@
##           master     #668   +/-   ##
=======================================
  Coverage   83.87%   83.87%           
=======================================
  Files         247      247           
  Lines       20401    20376   -25     
  Branches     2757     2748    -9     
=======================================
- Hits        17111    17091   -20     
+ Misses       2711     2710    -1     
+ Partials      579      575    -4     

Impacted Files	Coverage Δ
hathor/p2p/sync_v1/downloader.py	83.33% <ø> (ø)
hathor/manager.py	77.21% <76.92%> (+3.24%)	⬆️
hathor/p2p/manager.py	72.51% <100.00%> (ø)
hathor/p2p/sync_v1/agent.py	85.58% <100.00%> (ø)
hathor/p2p/sync_v1/factory_v1_0.py	100.00% <100.00%> (ø)
hathor/p2p/sync_v1/factory_v1_1.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes