Audit issue 7: Remove obsolete outbound_fee parameter

[vgeddes]

Outbound fees on ethereum are now calculated dynamically using an exchange rate, and so this outbound_fee parameter is obsolete and should have been previously removed.

In parachain/pallets/system/src/lib.rs:441-465 and in parachain/pallets/system/src/lib.rs:479-495, the update_channel and force_update_channel extrinsics permit to modify channels by specifying the mode and the outbound_fee.
However, the outbound_fee is currently disregarded and not processed either in the pallet or within the Gateway contract; it is solely emitted in the event.
Despite being defined in the comment as the “Fee charged to users for sending outbound messages to Polkadot”, this specific fee per channel is not currently implemented.

[yrong]

LGTM!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6f9cfdd) 81.29% compared to head (fc20079) 81.30%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1051   +/-   ##
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  Coverage   81.29%   81.30%           
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  Files          54       54           
  Lines        2240     2241    +1     
  Branches       71       71           
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+ Hits         1821     1822    +1     
  Misses        402      402           
  Partials       17       17           

Flag	Coverage Δ
rust	81.46% <100.00%> (+0.01%)	⬆️

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