Drop useless mut in KeysInterface::sign_holder_anchor_input

[TheBlueMatt]

The Transaction is not modified so there's no reason to make the reference mut

[codecov-commenter]

Codecov Report

Base: 90.72% // Head: 90.72% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (f63df16) compared to base (2e343e7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1805      +/-   ##
==========================================
- Coverage   90.72%   90.72%   -0.01%     
==========================================
  Files          87       87              
  Lines       47364    47364              
  Branches    47364    47364              
==========================================
- Hits        42970    42969       -1     
- Misses       4394     4395       +1     

Impacted Files	Coverage Δ
lightning/src/chain/keysinterface.rs	89.40% <ø> (ø)
lightning/src/util/enforcing_trait_impls.rs	86.17% <ø> (ø)
lightning/src/chain/onchaintx.rs	95.06% <0.00%> (-0.23%)	⬇️
lightning/src/ln/functional_tests.rs	96.94% <0.00%> (-0.03%)	⬇️
lightning/src/ln/channelmanager.rs	85.18% <0.00%> (+0.02%)	⬆️
lightning/src/ln/monitor_tests.rs	99.55% <0.00%> (+0.11%)	⬆️

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