Refactor: rename rmSpace to remove_redundant_spaces (#10796)

### What problem does this PR solve?

- rename rmSpace to remove_redundant_spaces
- move clean_markdown_block to common module
- add unit tests for remove_redundant_spaces and clean_markdown_block

### Type of change

- [x] Refactoring

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Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/app/picture.py

@@ -24,7 +24,7 @@ from api.db import LLMType
 from api.db.services.llm_service import LLMBundle
 from deepdoc.vision import OCR
 from rag.nlp import rag_tokenizer, tokenize
-from rag.utils import clean_markdown_block
+from common.string_utils import clean_markdown_block
 
 ocr = OCR()