Remove "New Tab" shortcut Ctrl+Shift+N

Since https://commits.kde.org/kconfig/782e43f8076e5f4a10111a6b4d2cc6b1741c9798 Ctrl+Shift+N is the standard shortcut for creating a new folder which leads to a shortcut conflict when this key combination is invoked in Dolphin.

"New Tab" already has the more popular shortcut Ctrl+T assigned to it, so Ctrl+Shift+N can simply be removed.