[AzureLoadTesting] Remove Preview tag for Azure Load Testing

[venkatr21]

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This checklist is used to make sure that common guidelines for a pull request are followed.

Related command

General Guidelines

• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

For new extensions:

• My extension description/summary conforms to the Extension Summary Guidelines.

About Extension Publish

There is a pipeline to automatically build, upload and publish extension wheels.
Once your pull request is merged into main branch, a new pull request will be created to update src/index.json automatically.
The precondition is to put your code inside this repository and upgrade the version in the pull request but do not modify src/index.json.

[yonzhan]

AzureLoadTesting

[azclibot]

[Release] Update index.json for extension [ load ] : https://dev.azure.com/azclitools/internal/_build/results?buildId=30058&view=results