Revert "Zero marks by GDEF for Tibetan"

This reverts commit d5bd0590ae2fbc7b0dee86385a565aef00ffb835.

The reasoning behind that logic was flawed and made under
a misunderstanding of the original problem, and caused
regressions as reported by Jonathan Kew in thread titled
"tibetan marks" in Oct 2013.  Apparently I have had fixed
the original problem with this commit:

  7e08f1258da229dfaf7e1c4b5c41e5bb83906cb0

So, revert the faulty commit and everything seems to be in good
shape.