Cleanup phantom QPA's handling of the font database on non-mac unix.

We now use the fontdatabases.pri include to setup everything required
in that regard, similar to how qnx.pro is doing it. Furthermore we
instantiate QGenericUnixFontDatabase instead of QFontconfigDatabase
directly. This is a typedef to QFontconfigDatabase if available, and
will otherwise fall-back to QBasicFontDatabase.

Change-Id: Icfad09d0328d20c9abcf23bfc05574c51b84ba53