This PR aims to remove duplicate embedded images taken by `PDFminer`.
### Summary
- add `clean_pdfminer_duplicate_image_elements()` to remove embedded
images with similar `bboxes` and the same `text`
- add env_config `EMBEDDED_IMAGE_SAME_REGION_THRESHOLD` to consider the
bounding boxes of two embedded images as the same region
- refactor: reorganzie `clean_pdfminer_inner_elements()`
