feat(BUX-180): remove option to use whatsonchain and nownodes for broadcasting

[witalij-4chain]

Context:

1. As I understood from previous discussions we should focus on providing support for common/standarized API especially Arc (and for now still support for mApi).

2. Just as an extension to that, we have similar broadcasting support for Now Nodes, should we remove it also?

Answers:

1. Agreed

2. Agreed, we are specifically building on this software to avoid relying on a node. NowNodes is counter to this mission.

[codecov]

Codecov Report

Merging #389 (a4705b7) into master (116cc85) will decrease coverage by 0.40%.
Report is 4 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #389      +/-   ##
==========================================
- Coverage   52.48%   52.08%   -0.40%     
==========================================
  Files          92       92              
  Lines       10800    10749      -51     
==========================================
- Hits         5668     5599      -69     
- Misses       4694     4714      +20     
+ Partials      438      436       -2     

Flag	Coverage Δ
unittests	52.08% <ø> (-0.40%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
chainstate/broadcast.go	88.15% <ø> (-1.73%)	⬇️
chainstate/broadcast_providers.go	49.01% <ø> (-15.03%)	⬇️

... and 1 file with indirect coverage changes

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[mrz1836]

LGTM