Comment out gpu metric gathering code that cannot succeed

[ClifHouck]

Since it can never be reached. See #321

[ClifHouck]

@rmccorm4 Hopefully this addresses your last comment on #342.

[rmccorm4]

Closing this in favor of moving forward with the original proposed changes: https://github.com/triton-inference-server/core/pull/321#issuecomment-2263784011