Remove unused async decoder preallocations

[KazukiKomon]

Summary

Remove unused cuMemAllocAsync preallocations from NvDecoder::HandleVideoSequence and NvDecoder::ReconfigureDecoder.

These local pFrame[8] allocations are not stored in m_vpFrame, so they are not managed by the decoder frame lifecycle and cannot be released by the destructor. Output frame buffers are already allocated lazily in HandlePictureDisplay and tracked by m_vpFrame.

This is intended as a small cleanup/risk reduction before enabling bEnableAsyncAllocations in GOP decode paths.

Verification

• git diff --check

[KazukiKomon]

Closing this draft because it only covered a cleanup follow-up, not the intended internal MR diff.