[misc] refactor: clean up unused sharding managers

[ji-huazhong]

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[gemini-code-assist]

Code Review

This pull request is a refactoring that cleans up several unused and deprecated sharding managers: FSDPVLLMShardingManager, MegatronSGLangShardingManager, and MegatronVLLMShardingManager. The corresponding files have been removed, and documentation has been updated to reflect these changes. The changes are straightforward and appear to be correct. I have not found any remaining references to the removed components in the provided code context. The PR contributes to better code maintainability by removing obsolete code.