refactor(gemini): remove GeminiVisionMaxImageNum constant and related image count logic

constant/env.go

@@ -11,7 +11,6 @@ var GetMediaTokenNotStream bool
 var UpdateTask bool
 var MaxRequestBodyMB int
 var AzureDefaultAPIVersion string
-var GeminiVisionMaxImageNum int
 var NotifyLimitCount int
 var NotificationLimitDurationMinute int
 var GenerateDefaultToken bool