[VitisAI] fix deinit vitisai ep (#23725)
### Description
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Removed the schema when unloading.
### Motivation and Context
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- If it fixes an open issue, please link to the issue here. -->
This fix the crash where the onnxruntime reloads vitisai ep.
Co-authored-by: Yueqing Zhang <yueqingz@amd.com>
