New answers tagged metacity
Hold Alt while right-mouse clicking on the said icon. A menu should then appear giving you the options to either move it or remove from panel. If it's not worked use both of Logo+Alt. the key Logo is win logo key on your keyboard.
Answering to myself, I finally managed to change the shortcuts using the dconf-editor, the shortcuts are stored in the key folder: org.gnome.desktop.wm.keybindings.
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