Remove obsolete field.

This field was removed from libgit2 a year and a half ago.

Author: txdv

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;