Remove anthropic.thinking.budgetTokens setting and hardcode budget to 16000 (#312824)

Remove the user-configurable setting github.copilot.chat.anthropic.thinking.budgetTokens
and hardcode the thinking budget to 16000 (the previous default value).

This simplifies the thinking budget logic by:
- Removing the setting definition from ConfigKey, package.json, and NLS
- Hardcoding budget=16000 in messagesApi.ts and anthropicProvider.ts
- Removing the ability to disable thinking by setting budget to 0
- Cleaning up unused imports/variables in the test file