fix: remove unsupported Voice & Audio Assistant

[calebeden]

Resolves #1062

Reverts #800.

Additional instructions

MUST: Keep Allow edits from maintainers enabled for this PR so maintainers
can help update the branch when needed.

What Problem This Solves

Fixes an issue where users starting Assistant from Voice & Audio were directed to provide an unbundled developer backend checkout that is not included with the Windows installer.

Why This Change Was Made

Removes the unsupported Assistant surface and its local backend bridge. Adding Assistant back as a first-class, packaged feature with a supported setup and configuration experience remains future work.

User Impact

Users no longer encounter an Assistant control that cannot work in a normal installed configuration. Existing speech-to-text, text-to-speech, and voice settings remain available.

Evidence

• The repository builds successfully after removing the Assistant integration.
• No Assistant bridge, backend environment variable, or Voice Settings Assistant resource references remain.
• Shared and Tray test suites pass.

Change Type

• Bug fix
• Feature
• Refactor
• Docs or instructions
• Tests or validation
• Security hardening
• Chore or infrastructure

Scope

• Tray or WinUI UX
• Windows node capability
• Local MCP or winnode
• Gateway, connection, or pairing
• Setup or onboarding
• Permissions, privacy, or security
• Tests, CI, or docs

Validation

• .\build.ps1: passed
• dotnet test .\tests\OpenClaw.Shared.Tests\OpenClaw.Shared.Tests.csproj: 3,221 passed, 31 skipped
• dotnet test .\tests\OpenClaw.Tray.Tests\OpenClaw.Tray.Tests.csproj --no-restore: 1,948 passed
• dotnet test .\tests\OpenClaw.Tray.Tests\OpenClaw.Tray.Tests.csproj --filter FullyQualifiedName~LocalizationValidationTests: 15 passed

Real Behavior Proof

• Environment tested: Windows 11 ARM64
• PR head or commit tested: 5e8a07cd0081bb97a18af05954bd1705e3831754
• Exact steps or command run: Built the repository, ran Shared and Tray tests, and searched tracked files for the removed Assistant bridge identifiers and backend commands.
• Evidence after fix: No matching Assistant bridge or backend command references remain.
• Observed result: Voice & Audio retains its supported speech settings without the unsupported Assistant card or process-launching bridge.
• Screenshot or artifact links verified? (Yes/No/N/A): N/A
• Not verified or blocked: No visual screenshot captured.

Security Impact

• New permissions or capabilities? (Yes/No): No
• Secrets or tokens handling changed? (Yes/No): No
• New or changed network calls? (Yes/No): No
• Command or tool execution surface changed? (Yes/No): Yes
• Data access scope changed? (Yes/No): No
• If any answer is Yes, explain the risk and mitigation: Removes the unsupported local backend process-launching surface.

Compatibility and Migration

• Backward compatible? (Yes/No): No
• Config or environment changes? (Yes/No): Yes
• Migration needed? (Yes/No): No
• If yes, list the exact upgrade steps: None. OPENCLAW_BACKEND_ROOT is no longer read by the Windows app.

Review Conversations

• I replied to or resolved every bot review conversation addressed by this PR.
• I left unresolved only conversations that still need maintainer judgment.