Q:

Application icon appearing in the corner of window

I was trying to use Elementary interface in my Ubuntu. I also installed elementary-tweaks. I removed everything but I don't know if happened after removed.
Here goes what is happening, I didn't explain well.

A:

If you want to get rid of the small application icon next to the close button from the title-bar, you can do that by changing your window button-layout. Open Terminal and run
gsettings set org.gnome.desktop.wm.preferences button-layout :minimize,maximize,close

Alternatively, you can have the app-menu in the top bar instead. You may use (GNOME) Tweaks. First install it (if it is not installed) by running 
sudo apt install gnome-tweaks

Then launch Tweaks, go to "Top Bar" section and toggle on the "Application Menu" option.

