[Image Update] Remove prerelease for powershell predictor

[dsajanice]

Thank you for filing this request! Please take a few minutes to fill out this template so that we can better understand the request.

Which image would you like to change?

The powershell setup script includes a prerelease flag for the PSReadLine utlity. PSReadLine now includes predictor by default, so we need to take out the prerelease flag and install the latest PSReadLine module to get predictor capability.

What changes are you requesting?

What scenario are these changes enabling?

By how much does this change change the image size?

[theJasonHelmick]

This is planned as a future update in October.

[theJasonHelmick]

The latest version of PSReadLine, with predictors enabled by default, are included in the new image pr. This pr also updates and enables the az.tools.predictor. This issue will be fixed with its release. https://github.com/Azure/CloudShell/pull/252

[theJasonHelmick]

This is now removed with the release of #252.