Open WhatsApp
Go to the chat where you want to remove the wallpaper
Tap the three dots menu
Tap Wallpaper
Tap Change
Select No Wallpaper or Default Wallpaper
Tap Set Wallpaper if prompted
Repeat for other chats if needed
To remove wallpaper from all chats, go to WhatsApp Settings
Tap Chats
Tap Wallpaper
Select No Wallpaper or Default Wallpaper
Confirm the change