How do you delete that game pigeon app

See "Manage iMessage Apps" here:

Use iMessage apps on your iPhone, iPad, and iPod touch - Apple Support

Press (not too firmly) and hold the application until it wiggles. Press the "X" in the upper left corner to delete the application.

End the wiggling by pressing the home button (unless you have an iPhone X, in which case you would swipe up from the bottom instead).