fix: remove empty PDF from benchmark fixtures

Removed issue-297-example.pdf fixture as it contains no extractable
content (truly empty PDF). Benchmarking empty files provides no value
and only creates noise in results.

This PDF was correctly returning empty content, but there's no point
in benchmarking extraction performance on files with nothing to extract.

Removed:
- tools/benchmark-harness/fixtures/pdf/issue-297-example.json

Author: Goldziher