Remove manifest that no longer works for Doxygen version 1.8.20 (#6234)

manifests/DimitriVanHeesch/Doxygen/1.8.20.yaml

@@ -1,18 +0,0 @@
-Id: DimitriVanHeesch.Doxygen
-Version: 1.8.20
-Name: Doxygen
-Publisher: Dimitri van Heesch
-License: Copyright © 1997-2018 by Dimitri van Heesch.
-LicenseUrl: http://doxygen.nl/index.html
-AppMoniker: doxygen
-Tags: documentation, generator
-Description: Doxygen is the de facto standard tool for generating documentation from annotated C++ sources, but it also supports other popular programming languages such as C, Objective-C, C#, PHP, Java, Python, IDL (Corba, Microsoft, and UNO/OpenOffice flavors), Fortran, VHDL and to some extent D.
-Homepage: http://www.doxygen.nl/index.html
-InstallerType: inno
-Installers:
-  - Arch: x86
-    Url: http://doxygen.nl/files/doxygen-1.8.20-setup.exe
-    Sha256: 30f19774d7cbceddfb678bb411b969b4f3d370ecc5c56f268e40e6b3049c07a7
-    Switches:
-      Silent: /verysilent /norestart
-      SilentWithProgress: /silent /norestart