gnu: Delete mono and dependent packages.

Fixes <https://issues.guix.gnu.org/55026>.

* gnu/packages/mono.scm: Delete file.
* gnu/packages/patches/mono-mdoc-timestamping.patch: Likewise.
* gnu/local.mk (GNU_SYSTEM_MODULES): De-register module file.
(dist_patch_DATA): Re-register patch file.
* gnu/packages/game-development.scm (fna): Delete variable.
* gnu/packages/gl.scm (mojoshader-cs): Likewise.
* gnu/packages/sdl.scm (sdl2-cs): Likewise.

gnu/packages/gl.scm

@@ -48,7 +48,6 @@
   #:use-module (gnu packages image)
   #:use-module (gnu packages linux)
   #:use-module (gnu packages llvm)
-  #:use-module (gnu packages mono)
   #:use-module (gnu packages pkg-config)
   #:use-module (gnu packages python)
   #:use-module (gnu packages python-xyz)
@@ -1043,44 +1042,6 @@ the shaders at runtime.")
 the glProgramViewportFlip before it was replaced with glProgramViewportInfo.")
       (license license:zlib))))
 
-(define-public mojoshader-cs
-  (let ((commit "10d0dba21ff1cfe332eb7de328a2adce01286bd7"))
-    (package
-      (name "mojoshader-cs")
-      (version (git-version "20191205" "1" commit))
-      (source (origin
-                (method git-fetch)
-                (uri (git-reference
-                      (url "https://github.com/FNA-XNA/MojoShader")
-                      (commit commit)))
-                (file-name (git-file-name name version))
-                (sha256
-                 (base32
-                  "11mdhf3fmb9rsn2iv753gmb596j4dh5j2iipgw078vg0lj23rml7"))))
-      (build-system gnu-build-system)
-      (arguments
-       '(#:tests? #f  ; No tests.
-         #:phases
-         (modify-phases %standard-phases
-           (delete 'configure)
-           (replace 'build
-             (lambda _
-               (invoke "make" "-C" "csharp")))
-           (replace 'install
-             (lambda* (#:key outputs #:allow-other-keys)
-               (let ((out (assoc-ref outputs "out")))
-                 (install-file "csharp/bin/MojoShader-CS.dll" (string-append out "/lib"))
-                 #t))))))
-      (native-inputs
-       (list mono))
-      (home-page "https://github.com/FNA-XNA/MojoShader")
-      (synopsis "C# wrapper for MojoShader")
-      (description
-       "Mojoshader-CS provides C# bindings for the Mojoshader library.
-The C# wrapper was written to be used for FNA's platform support.  However, this
-is written in a way that can be used for any general C# application.")
-      (license license:zlib))))
-
 (define-public glmark2
   (package
     (name "glmark2")