Remove obsolete field.

txdv · txdv · commit 67626b723932 · 2016-07-13T14:17:48.000+03:00
This field was removed from libgit2 a year and a half ago.
diff --git a/LibGit2Sharp/Core/GitCloneOptions.cs b/LibGit2Sharp/Core/GitCloneOptions.cs
@@ -23,8 +23,6 @@ internal struct GitCloneOptions
         public GitCloneLocal Local;
         public IntPtr CheckoutBranch;
 
-        public IntPtr signature; // Really a SignatureSafeHandle
-
         public IntPtr RepositoryCb;
         public IntPtr RepositoryCbPayload;
 
