Remove version test to avoid maintenance burden

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kako-jun

diffx-cli/tests/cmd/output.md

@@ -73,10 +73,3 @@ $ diffx --quiet tests/fixtures/file1.json tests/fixtures/file2.json
 
 ```
 
-## Version
-
-```console
-$ diffx --version
-diffx 0.6.0
-
-```