Remove check for File.fileSeparatorChar from MKD command. Will fail incorrectly on custom file systems, and will disallow creation of directories which the OS allows in some cases (FTPSERVER-348)

git-svn-id: https://svn.apache.org/repos/asf/mina/ftpserver/trunk@917039 13f79535-47bb-0310-9956-ffa450edef68