(Minor) Remove an unused import (ChannelSigner)

[optout21]

One-liner fix for an unused import warning.
Remove unused import sign::ChannelSigner.

Rids the following compiler warning:

warning: unused import: `ChannelSigner`
  --> lightning/src/events/bump_transaction.rs:29:19
   |
29 | use crate::sign::{ChannelSigner, EcdsaChannelSigner, SignerProvider, WriteableEcdsaChannelSigner};
   |                   ^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.02% ⚠️

Comparison is base (685f266) 90.33% compared to head (1babca4) 90.32%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2454      +/-   ##
==========================================
- Coverage   90.33%   90.32%   -0.02%     
==========================================
  Files         106      106              
  Lines       55732    55732              
  Branches    55732    55732              
==========================================
- Hits        50347    50338       -9     
- Misses       5385     5394       +9     

Files Changed	Coverage Δ
lightning/src/events/bump_transaction.rs	77.55% <ø> (ø)

... and 3 files with indirect coverage changes

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[optout21]

One test failure, seems to be unrelated to the change.

[TheBlueMatt]

Thanks!