Remove "Close session" from context menu
When terminal display's context menu opens above mouse cursor, this
entry is really easy to click accidentally (mouse button down, move
cursor 2 pixels, mouse button up) and in effect close the session.
Moreover, the option is not really needed here. To close the session
using mouse, user can click window's close button, use tab context
menu, tab close button, or menu entry. Other tab/view-based applications
(e.g. Chromium, Firefox, Kate) do not have close button in view context
menu.