Remove unnecessary mut from reference

[jkczyz]

chain::Listen's interface is defined in terms of non-mutable references, so there's no need to use mutable references in lightning-block-sync.

[codecov-commenter]

Codecov Report

Merging #1304 (494fe6d) into main (c931380) will increase coverage by 0.36%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1304      +/-   ##
==========================================
+ Coverage   90.50%   90.87%   +0.36%     
==========================================
  Files          71       71              
  Lines       39037    40584    +1547     
==========================================
+ Hits        35329    36879    +1550     
+ Misses       3708     3705       -3     

Impacted Files	Coverage Δ
lightning-block-sync/src/init.rs	93.56% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.22% <0.00%> (+0.01%)	⬆️
lightning/src/util/config.rs	47.82% <0.00%> (+2.17%)	⬆️
lightning/src/ln/channel.rs	92.52% <0.00%> (+3.21%)	⬆️

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