Remove unsupported type_comment field

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## Summary

This PR removes the `type_comment` field which our parser doesn't support.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

`cargo test`

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crates/ruff_python_formatter/src/other/arg.rs

@@ -12,7 +12,6 @@ impl FormatNodeRule<Arg> for FormatArg {
             range: _,
             arg,
             annotation,
-            type_comment: _,
         } = item;
 
         arg.format().fmt(f)?;

crates/ruff_python_formatter/src/statement/stmt_assign.rs

@@ -17,7 +17,6 @@ impl FormatNodeRule<StmtAssign> for FormatStmtAssign {
             range: _,
             targets,
             value,
-            type_comment: _,
         } = item;
 
         let (first, rest) = targets.split_first().ok_or(FormatError::syntax_error(