chore: remove unnecessary using to fix the Lint check

[lahma]

The Lint check (dotnet format --no-restore --verify-no-changes) is currently failing on main and therefore on every open pull request. This removes the one directive it flags.

test/Docfx.Dotnet.Tests/SymbolUrlResolverUnitTest.cs(5,1): warning IDE0005: Using directive is unnecessary.

Why the directive is genuinely unnecessary

No name written in that file resolves through Microsoft.CodeAnalysis. The symbol types only ever appear bound to var — CompilationHelper.CreateCompilationFromAssembly is deconstructed into var (compilation, assembly), and everything afterwards (assembly.GetTypeByMetadataName, type.GetMembers) is an instance member call, which needs no using. The file compiles unchanged without it.

Evidence that this is pre-existing rather than PR-specific

• A clean worktree of main at 2a436495ed reproduces the failure exactly: dotnet format --no-restore --verify-no-changes exits 2 with that single warning and nothing else.
• The same failure is on chore(deps-dev): bump js-yaml from 4.1.1 to 4.2.0 in /templates #11081, chore(deps): bump dompurify from 3.3.1 to 3.4.11 in /templates #11080, fix(deps): update esbuild to 0.28.1 and tsx to 4.22.4 #11077 and chore(deps): bump uuid and mermaid in /templates #11073 — the file belongs to Docfx.Dotnet.Tests, which none of them touch.
• An older PR, deps: Update playwright version to 1.60.0 #11074, passed Lint, so the diagnostic started appearing at some point independent of any change in the repository.

Verification

• dotnet format --no-restore --verify-no-changes — the exact CI command — now exits 0 across the whole solution.
• Docfx.Dotnet.Tests, Release, net10.0: 172/172 passing, 0 build warnings.

A durable fix worth considering separately

The repository pins no global.json, so the analyzer set is whatever SDK the runner image happens to ship. IDE0005's analysis got stricter in a newer Roslyn and the check went red without anything in the repository changing — which is a failure mode that will recur on the next image bump. Pinning an SDK version in global.json (with rollForward set to taste) would make the lint result reproducible rather than dependent on runner timing. I have left that out of this PR, since removing the directive is what unblocks CI today and the pin is a separate decision for you to make.