fix: remove redundant collect_extra_allowed_attributes call

[ANtutov]

Summary

Removed the redundant collect_extra_allowed_attributes call from validate_attributes_flat and use the passed extra_allowed_attributes parameter directly. This eliminates duplicate work while maintaining the same validation behavior.

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Type of change

• Style, wording, formatting, or typo-only change

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Why is this change needed?

The validate_attributes_flat function redundantly called collect_extra_allowed_attributes internally, even though the caller validate_attributes already collected these attributes and passed them as the extra_allowed_attributes parameter. This caused:

1. Double work for top-level items - attributes were collected twice
2. Unnecessary calls for nested items (trait functions, struct fields, enum variants) which cannot have #[allow(...)] attributes anyway

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[reviewable-StarkWare]

This change is 

[orizi]

@orizi made 1 comment.
Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @ANtutov).

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a discussion (no related file):
The PR assumes nested items can't have #[allow_attr(...)], but removing the local collection prevents nested items from defining their own allowed attributes. This could be a breaking change if any nested items use #[allow_attr(...)].