I'm not sure why the nvptx offloading plugin introduced struct map in
the first place, but its current usage is both unnecessary and buggy.
For instance, it doesn't properly free data mappings, and that results
in bogus duplicate data mapping errors. This patch removes that struct
altogether.

Jim originally posted this patch back in December, but it looks like it
never made it's way into trunk. This patch is in gomp4 though, but I had
to rebase it to trunk because trunk contains some runtime async changes
that aren't in gomp4 yet. Here's the posting to the original patch
<https://gcc.gnu.org/ml/gcc-patches/2015-12/msg01658.html>.

Is this OK for trunk?

Cesar