Remove emitting of ConvertParseThreeError::SelfImplForContract

[ironcev]

Description

This PR removes the ConvertParseThreeError::SelfImplForContract and the unreachable code branch that was emiting it.

#7275 introduces self impls for contracts and transforms every impl Contract into impl <GeneratedAbiName> for Contract which makes the SelfImplForContract error obsolete and the match arm that was emitting it unreachable.

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #7375 will improve performances by 20.9%

_{Comparing ironcev/remove-self-impl-for-contract-error (4d2822f) with master (1f9880f)}

Summary

⚡ 1 improvements
✅ 24 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	code_lens	5.8 µs	4.8 µs	+20.9%