Duplicate startup warnings in admin panel

[silverwind]

Description

StartupProblems warnings (from deprecatedSetting and other LogStartupProblem calls) appear twice in the admin panel at /-/admin/self_check and other places.

Cause

LoadCommonSettings() is called twice during web server startup:

1. Early init via cmd/main.go:79 → InitWorkPathAndCommonConfig → LoadCommonSettings()
2. Web server startup via cmd/web.go:167 → LoadCommonSettings()

The second call re-initializes the config provider first (InitCfgProvider), but the StartupProblems slice in modules/setting/config_provider.go is never cleared between loads, so every warning gets appended twice.

How to reproduce

1. Add a deprecated setting to app.ini, e.g. [oauth2] → ENABLE = true (deprecated in favor of ENABLED)
2. Start Gitea
3. Go to Site Administration → the warnings section shows each warning duplicated