Convert send_channel_ready macro to method

[elnosh]

Don't think this needs to be macro.

[ldk-reviews-bot]

I've assigned @joostjager as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 88.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.35%. Comparing base (18cebae) to head (3247fad).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	88.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4279      +/-   ##
==========================================
- Coverage   89.36%   89.35%   -0.01%     
==========================================
  Files         180      180              
  Lines      139827   139847      +20     
  Branches   139827   139847      +20     
==========================================
+ Hits       124956   124961       +5     
- Misses      12280    12297      +17     
+ Partials     2591     2589       -2     

Flag	Coverage Δ
fuzzing	35.21% <84.00%> (+0.01%)	⬆️
tests	88.69% <88.00%> (-0.01%)	⬇️

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[joostjager]

Let's continue the macro removal trend