refactor(core-p2p): remove unused NetworkMonitor.getServer()

[vasild]

A summary of what changes this PR introduces and why they were made.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactor
• Performance
• Tests
• Build
• Documentation
• Code style update
• Continuous Integration
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

Does this PR release a new version?

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch
• All tests are passing
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

[codecov-io]

Codecov Report

Merging #2829 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2829      +/-   ##
===========================================
+ Coverage    65.34%   65.34%   +<.01%     
===========================================
  Files          363      363              
  Lines         8194     8193       -1     
  Branches       392      428      +36     
===========================================
  Hits          5354     5354              
+ Misses        2806     2801       -5     
- Partials        34       38       +4

Impacted Files	Coverage Δ
packages/core-p2p/src/network-monitor.ts	69.06% <ø> (+0.37%)	⬆️
packages/core-api/src/handlers/utils.ts	76% <0%> (ø)	⬆️
packages/core-logger-winston/src/formatter.ts	41.66% <0%> (ø)	⬆️
...es/core-blockchain/src/replay/replay-blockchain.ts	0% <0%> (ø)	⬆️

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[alessiodf]

Will this prevent plugins from being able to add additional websocket endpoints (may have been useful for bridgechains to do this via plugins rather than modifying the core code)?

[spkjp]

Given that it can be useful for plugins, we should keep it.