revert: completely remove all Sora functionality

backend/internal/handler/admin/account_handler.go

@@ -1875,12 +1875,6 @@ func (h *AccountHandler) GetAvailableModels(c *gin.Context) {
 		return
 	}
 
-	// Handle Sora accounts
-	if account.Platform == service.PlatformSora {
-		response.Success(c, service.DefaultSoraModels(nil))
-		return
-	}
-
 	// Handle Claude/Anthropic accounts
 	// For OAuth and Setup-Token accounts: return default models
 	if account.IsOAuth() {