DolphinContextMenu: drop wrong assert

`m_context` can also be `SearchContext` or `TimelineContext`, so this
assert has been wrong since 8b0c12a5.

src/dolphincontextmenu.cpp

@@ -103,7 +103,6 @@ DolphinContextMenu::Command DolphinContextMenu::open()
     } else if (m_context & ItemContext) {
         openItemContextMenu();
     } else {
-        Q_ASSERT(m_context == NoContext);
         openViewportContextMenu();
     }