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/Graphics/FramebufferDevice.cpp

@@ -84,11 +84,6 @@ void FramebufferDevice::activate_writes()
     m_graphical_writes_enabled = true;
 }
 
-String FramebufferDevice::device_name() const
-{
-    return String::formatted("fb{}", minor());
-}
-
 UNMAP_AFTER_INIT KResult FramebufferDevice::initialize()
 {
     // FIXME: Would be nice to be able to unify this with mmap above, but this