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/Devices/SerialDevice.cpp

@@ -104,11 +104,6 @@ void SerialDevice::put_char(char ch)
     m_last_put_char_was_carriage_return = (ch == '\r');
 }
 
-String SerialDevice::device_name() const
-{
-    return String::formatted("ttyS{}", minor() - 64);
-}
-
 UNMAP_AFTER_INIT void SerialDevice::initialize()
 {
     set_interrupts(false);