Kernel: Rename File::{before_removing => will_be_destroyed}

This will allow File and it's descendants to use RefCounted instead of
having a custom implementation of unref. (Since RefCounted calls
will_be_destroyed automatically)

This commit also removes an erroneous call to `before_removing` in
AHCIPort, this is a duplicate call, as the only reference to the device
is immediately dropped following the call, which in turns calls
`before_removing` via File::unref.

Kernel/Storage/ATA/AHCIPort.cpp

@@ -77,7 +77,6 @@ void AHCIPort::handle_interrupt()
             m_connected_device->prepare_for_unplug();
             StorageManagement::the().remove_device(*m_connected_device);
             g_io_work->queue([this]() {
-                m_connected_device->before_removing();
                 m_connected_device.clear();
             });
         } else {