Stop exporting QT_BINDIR and QT_LIBDIR to PLIST_SUB.

As at the moment QT_BINDIR and QT_LIBDIR are 'bin' respectively 'lib' depending
on the Qt version these subs ended up at many wrong places in plists.

So only export it if QT_DIST is set.

PR: 210227
Reviewed by: mat, rakuco
Approved by: portmgr (mat), rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D8378