japanese/open_jtalk: fix pkg-plist

pkg-plist hardcoded a framework-managed line:
@dir share/licenses/ja-open_jtalk-1.11

share/licenses/<pkg>/ is created automatically by the LICENSE
framework and must never be in pkg-plist.

Putting that line causes a packaging failure when DISABLE_LICENSES=yes
is set:
pkg-static: Unable to access file /wrkdirs/usr/ports/japanese/open_jtalk/work/stage/usr/local/share/licenses/ja-open_jtalk-1.11/:No such file or directory