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MINOR: [C++] Remove wrong comment in swiss join avx2 (#45107)



### Rationale for this change

A piece of comment is wrongly introduced in #43389.

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

Authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>