IOS/ES: Move ImportTicket write function

This commit moves the write function to where it should be (IOS),
especially when ES::ImportTicket() is the only place to use it.

Prevents misusing the ticket import function, and removes one unsafe
direct write to the NAND that does not go through IOS.

This also fixes the destination path: the session root is the one which
should be used for determining the ticket path, not the configured one.

Source/Core/DiscIO/NANDContentLoader.h

@@ -24,7 +24,6 @@ namespace DiscIO
 enum class Region;
 
 // TODO: move some of these to Core/IOS/ES.
-bool AddTicket(const IOS::ES::TicketReader& signed_ticket);
 IOS::ES::TicketReader FindSignedTicket(u64 title_id);
 
 class CNANDContentData