Revert "build: add .pc file for Banshee.NotificationArea extension"

This reverts commit e8c7720c8ac66f3a9aec2e5c75d50989df70f12d, because
the reference was actually not needed, see:

https://gitorious.org/banshee-community-extensions/banshee-community-extensions/commit/3e5999099055b7fc01635071222a84c37c873276