Commit being reverted is:
commit 689168be
Author: Vladislav Vaintroub <wlad@mariadb.com>
Date:   Thu Nov 16 18:57:18 2017 +0000

    MDEV-13852 - redefine WinWriteableFile such as IsSyncThreadSafe()
    is set to true, as it should.

    Copy and modify original io_win.h header file to a different location
    (as we cannot patch anything in submodule). Make sure modified header is
    used.