fix and simplify rust solution for `0036-valid-sudoku`

[Adimac93]

• File(s) Modified: 0036-valid-sudoku.rs
• Language(s) Used: rust
• Submission URL: https://leetcode.com/problems/valid-sudoku/submissions/949636309/

Previous incorrect submission: https://leetcode.com/problems/valid-sudoku/submissions/949595521/

1. fix for bx indices
2. use of returned insertion status

Note https://doc.rust-lang.org/std/collections/struct.HashSet.html#method.insert If the set did not previously contain this value, true is returned. If the set already contained this value, false is returned.