gnu: libgit2: Remove version 0.28.

This is a followup to e0e23164202ad9328ffeadde5e900857cb008124, which
removed "libgit2-mtime-0.patch", which was still used by LIBGIT2-0.28.

* gnu/packages/version-control.scm (libgit2-0.28): Remove.

gnu/packages/version-control.scm

@@ -850,24 +850,6 @@ write native speed custom Git applications in any language with bindings.")
     ;; GPLv2 with linking exception
     (license license:gpl2)))
 
-(define-public libgit2-0.28
-  (package
-    (inherit libgit2)
-    (version "0.28.5")
-    (source
-      (origin
-        (method url-fetch)
-        (uri (string-append "https://github.com/libgit2/libgit2/releases/"
-                            "download/v" version
-                            "/libgit2-" version ".tar.gz"))
-        (sha256
-         (base32
-          "0hjgpqjjmkciw1i8jqkx9q2vhdc4fc99qajhrj2bq8ziwsp6hyrb"))
-        (patches (search-patches "libgit2-mtime-0.patch"))
-        (modules '((guix build utils)))
-        (snippet '(begin
-                    (delete-file-recursively "deps") #t))))))
-
 (define-public git-crypt
   (package
     (name "git-crypt")