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/FileSystem/File.h

@@ -75,7 +75,7 @@ class File
     , public Weakable<File> {
 public:
     virtual bool unref() const;
-    virtual void before_removing() { }
+    virtual void will_be_destroyed() { }
     virtual ~File();
 
     virtual ErrorOr<NonnullRefPtr<OpenFileDescription>> open(int options);