Remove unused ibc::handler module

[kevinji]

Closes: #XXX

Description

Delete the now-unused ibc::handler module.

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests.
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).

[codecov]

Codecov Report

Base: 71.30% // Head: 71.52% // Increases project coverage by +0.21% 🎉

Coverage data is based on head (88c7045) compared to base (088861b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #478      +/-   ##
==========================================
+ Coverage   71.30%   71.52%   +0.21%     
==========================================
  Files         126      125       -1     
  Lines       15935    15886      -49     
==========================================
  Hits        11363    11363              
+ Misses       4572     4523      -49     

Impacted Files	Coverage Δ
crates/ibc/src/lib.rs	100.00% <ø> (ø)

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[plafer]

good catch, seems like we forgot about that one