textproc/itstool: Remove problematic PKGNAMEPREFIX

textproc/meld was failing its build-depends phase due to the PKGNAMEPREFIX
of itstool being set to ${PYTHON_PKGNAMEPREFIX}. This caused trouble as
textproc/itstool does not use flavors and those two ports could not agree
on the Python version.

Approved by: mat (mentor), portmgr blanket (fix dependencies)