reflog: remove unused sys functions

The `git_reflog_entry__alloc` function is not actually defined, nor
used. Remove references to it in the headers. It is not clear why the
corresponding `__free` is, or should be, exported. Make it internal to
the library.

include/git2/sys/reflog.h

@@ -1,21 +0,0 @@
-/*
- * Copyright (C) the libgit2 contributors. All rights reserved.
- *
- * This file is part of libgit2, distributed under the GNU GPL v2 with
- * a Linking Exception. For full terms see the included COPYING file.
- */
-#ifndef INCLUDE_sys_git_reflog_h__
-#define INCLUDE_sys_git_reflog_h__
-
-#include "git2/common.h"
-#include "git2/types.h"
-#include "git2/oid.h"
-
-GIT_BEGIN_DECL
-
-GIT_EXTERN(git_reflog_entry *) git_reflog_entry__alloc(void);
-GIT_EXTERN(void) git_reflog_entry__free(git_reflog_entry *entry);
-
-GIT_END_DECL
-
-#endif