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.