Q: remove contact from a group chat

That function only works if all the participants in the group are using iOS devices. If anyone is using something else (e.g. Android), that function is not available.

If an Android user is involved in the chat, you can not leave the conversation.

http://osxdaily.com/2014/09/23/leave-group-message-chat-ios/

This feature is only supported in iMessage. You have to start a separate conversation without that contact in MMS.

Send a group message with your iPhone, iPad, or iPod touch - Apple Support