Remove logic for platform stlport gtest.

Remove logic for platform stlport gtest.

Now that the platforms stlport linked gtest is gone, we don't need the
logic for this any more. All platform tests will use libc++ (which has
been renamed to libgtest without a suffix).

Note that tests built with the NDK will still use stlport, but they
will use the NDK's stlport, not the platform's.

Bug: 15193147
Change-Id: Ifcb2489ed60c41e4ed028096b516c1ee643de907