Stop including libjawt in libawt_xawt

This is from a code review following the jigsaw source file restructuring:

* The directory ./unix/native/libjawt is included in libawt_xawt (and in libjawt, of course). This seems suspicious to me. There is just a single file with a single function, JAWT_GetAWT(), which is exported in libjawt (via a mapfile), but not in libawt_xawt. I believe it is a mistake to include it in libawt_xawt. This will need verification from someone on the AWT team.