don't use a different de-duplication algo for the library list in mingw prl files

don't use a different de-duplication algo for the library list in mingw prl files

i have no idea why that was done (the commit message says "not sure why
it works elsewhere"), but it makes no sense whatsoever, specifically
doing it only on mingw. probably some workaround, as usual. the
de-duplication is broken by design anyway.

This reverts commit 7a6302c2baf6861fdaf65992b71a7676859860c2.

Change-Id: I6edecaa062570e59eccd24d50919ba132e65a403
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>