FluentD output plugin: Remove an unused variable when processing chunks

[MrWong99]

I couldn't figure out why this variable is there. If I am missing any global variables (or something similar) feel free to close this PR without any notice.

What this PR does / why we need it:

Removes an unused variable to clean up the code a tiny bit.

[CLAassistant]

All committers have signed the CLA.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

We use a stalebot among other tools to help manage the state of issues in this project.
A stalebot can be very useful in closing issues in a number of cases; the most common
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[cyriltovena]

LGTM sorry for the delay.