Fix the character spacing error in the rendering of Label under Char Mode

[smallmain]

Due to the uncleaned Kerning information, all Label renderings using Char Mode will result in rendering errors due to reused Kerning.

How to reproduce the issue:

1. Create an empty project
2. Create a Label, using the BMFont in the attached file for rendering
3. Run the project, and the engine's built-in Stats character spacing is problematic

bmfont.zip

Screenshot:

(The first and second characters are too close.)

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.