It's not notifiable work. But it does have to be done safely - you can't leave the cable in the wall unless you disconnect it from the CU.
Is this not a potentially slightly complicated question? IF one disconnected the circuit from the CU (which is not notifiable work), then the 'circuit' would cease to be part of the electrical installation, so one presumably could then do whatever one wanted to do to any part of it without notification.
However, if the circuit remains connected at the CU, I presume that anything done in the vicinity of the removed shower unit (presumably in 'Zone 1') to make things safe would constitute "an alteration to an existing circuit in a special location" - and therefore would be notifiable. To not be notifiable, I presume that the 'making safe' would have to be done outside of the zones of the bathroom, possibly at an isolator/switch (on ceiling or outside of bathroom)?
Kind Regards, John