Kernel/Devices: Remove required_mode and device_name methods

These methods are no longer needed because SystemServer is able to
populate the DevFS on its own.

Device absolute_path no longer assume a path to the /dev location,
because it really should not assume any path to a Device node.

Because StorageManagement still needs to know the storage name, we
declare a virtual method only for StorageDevices to override, but this
technique should really be removed later on.

Kernel/TTY/SlavePTY.h

@@ -38,9 +38,6 @@ private:
     virtual StringView class_name() const override { return "SlavePTY"; }
     virtual KResult close() override;
 
-    // ^Device
-    virtual String device_name() const override;
-
     friend class MasterPTY;
     SlavePTY(MasterPTY&, unsigned index);