Remove ad-hoc FBX importer

This importer was the fruit of a lot of amazing reverse engineering
work by RevoluPowered, based on the original Assimp importer that was
introduced by fire.

While promising and well tuned for a specific type of FBX scenes, it
was found to have many flaws to support the many FBX exporters and
legacy models that Godot users want to use. As we currently lack a
maintainer to improve it, those issues are left unresolved and FBX
import is still sub-par in the current Godot releases.

After some experimentation, we're instead adding a new importer that
relies on Facebook's `fbx2gltf` command line tool to convert FBX to
glTF, so that we can then use our well-maintained glTF importer.
See godotengine#59653 and https://github.com/facebookincubator/FBX2glTF for details.

COPYRIGHT.txt

@@ -63,13 +63,6 @@ Copyright: 2011, Ole Kniemeyer, MAXON, www.maxon.net
  2014-2022, Godot Engine contributors.
 License: Expat and Zlib
 
-Files: ./modules/fbx/fbx_parser/
-Comment: Open Asset Import Library (FBX parser)
-Copyright: 2006-2020, assimp team
- 2007-2022, Juan Linietsky, Ariel Manzur.
- 2014-2022, Godot Engine contributors.
-License: BSD-3-clause
-
 Files: ./platform/android/java/lib/aidl/com/android/*
  ./platform/android/java/lib/res/layout/status_bar_ongoing_event_progress_bar.xml
  ./platform/android/java/lib/src/com/google/android/*