remove generated XML mime type

x-sgf.xml represents a *generated* XML mime type and not a proper XML
mime type definition, thus installing it in /usr/share/mime/text will
cause its removal at the next update-mime-database run.

Furthermore, there is no need to provide a mime type definition for
*.sgf files, since shared-mime-info has been providing
application/x-go-sgf for years.
Merge pull request #30 from pinotree/mime

remove generated XML mime type