Minor gossip.rs rustfmt cleanups

[TheBlueMatt]

Just a few minor updates to gossip.rs to reduce code verticality and simplify things a bit.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.63%. Comparing base (053281f) to head (bc5213d).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/routing/gossip.rs	95.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3341      +/-   ##
==========================================
- Coverage   89.66%   89.63%   -0.03%     
==========================================
  Files         126      126              
  Lines      103226   103168      -58     
  Branches   103226   103168      -58     
==========================================
- Hits        92555    92479      -76     
- Misses       7954     7968      +14     
- Partials     2717     2721       +4     

Flag	Coverage Δ
	89.63% <95.00%> (-0.03%)	⬇️

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[optout21]

LGTM