test: remove redundant test/assertion in QueryStringTest (#188)

The QueryStringTest file was redundant because:
- QueryString class only uses traits (CommentTrait, NameValueTrait)
- These traits are already tested by HeaderTest and ParamsTest
- Other tests use QueryString objects indirectly

Verified: tests pass, coverage maintained at 97.73%, infection passes
with 100% MSI.

Co-authored-by: Claude <noreply@anthropic.com>

Author: deviantintegral