Remove fileio from mmcv and use mmengine.fileio instead

[zhouzaida]

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Motivation

Remove fileio from mmcv and use mmengine.fileio instead

Modification

• Remove mmcv/fileio
• Remove docs/en/understand_mmcv/io.md and docs/zh_cn/understand_mmcv/io.md
• Replace fileio interfaces

BC-breaking (Optional)

yes

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.