Remove dead VSIX release scripts

[meziantou]

What

Deletes build/publish-vsix.ps1, build/update-version.ps1 and build/extension-manifest.json. Those were the only files in build/, so the directory goes away with them.

Why

Every path the two scripts write to is gone:

• src/Meziantou.Analyzer.Vsix/ was removed in adf9e87 ("Use GitHub Actions and Remove VSIX package"), which takes out both the .vsix payload publish-vsix.ps1 publishes and the source.extension.vsixmanifest that update-version.ps1 rewrites.
• src/Meziantou.Analyzer/Meziantou.Analyzer.csproj, the other file update-version.ps1 rewrites, no longer exists either — the analyzer is now Meziantou.Analyzer.roslyn<version>.csproj.

update-version.ps1 is also fully superseded: the CI workflow computes the version inline and passes /p:Version= to build and pack.

extension-manifest.json describes a marketplace extension that is no longer produced.

Beyond the hygiene, these are release scripts — they documented a second, abandoned distribution channel and a PAT-based publish flow, the kind of stale artifact that gets resurrected and run against a stale identity.

Notes for the reviewer

• Nothing invokes these scripts: no reference anywhere outside build/ itself, and no .ps1 invocation in .github/.
• The one nearby hit is <None Include="build/**" .../> in src/Meziantou.Analyzer.Pack/Meziantou.Analyzer.Pack.csproj. It is project-relative — dotnet msbuild -getItem:None resolves it solely to src/Meziantou.Analyzer.Pack/build/Meziantou.Analyzer.props, never the repo-root folder. Packing is unaffected.
• The .gitignore comment about build/ refers to **/packages/build/, not this folder.
• No build or test run: these files are not inputs to any project, so no compiled code or test is affected.
• Not something this PR can do — if a Visual Studio Marketplace PAT still exists for the old publish flow, it is worth revoking. No CI secret references one (the only secrets. use in the repo is GITHUB_TOKEN in close-issues.yml), so any surviving token would live in the Azure DevOps account rather than in this repo.