fix two warnings building NuGet vsix

[zivkan]

Bug

Fixes: local builds

Description

Fixes two issues:

• Opening the repo in Visual Studio, the Error List window gets an error saying the Microsoft.Web.XmlTransform.dll is missing. Updating our VS.Client targets to skip on design time build fixes this
• On CLI builds I was getting a warning that the VSSDK Build tools targets was being imported twice. Some other VSSDK package is now importing it automatically, so we no longer need to do it ourselves.

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests N/A
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc. N/A