Remove the unused WiX MSI project

[rodchristiansen]

Part of removing WiX from the windowsadmins repos.

build/msi/StartSet.wixproj, Package.wxs and build/msi/postinstall.ps1 are dead code. build.ps1 builds every package, the MSI included, through cimipkg from build-info.yaml and scripts/, and nothing references build/msi. The copy of postinstall.ps1 there had also drifted from scripts/postinstall.ps1 (93 lines against 172).

What changed

• Deleted build/msi.
• Removed its .gitignore entry.
• One comment in DataDirectoryGuard.cs no longer mentions a separate MSI. The package postinstall applies the same root ACL with icacls.

What stays the same

The shipped package is unchanged. cimipkg builds it with identifier com.github.windowsadmins.startset, and scripts/postinstall.ps1 still installs the service, PATH and ACLs. No release ever shipped a WiX MSI: every release since v2026.06.11 ships only startset-x64.zip and startset-arm64.zip, which I checked through the releases API. So the WiX UpgradeCode 7E4F8A2D-… needs no supersedes entry.

Tested

• dotnet test StartSet.sln -c Release on a Windows 11 26200 x64 build host: 303 passed, 0 failed.
• git grep -i wix and git grep build/msi find nothing in the tree.
• Release assets were listed for every tag with gh api repos/windowsadmins/startset/releases. They contain no MSI.

Not tested

• A package build and install. The PR touches no build input: build.ps1, build-info.yaml and scripts/ are byte-identical to main. Installing would also hit the VBScript limitation in Sandbox 26100 (error 2738).