libglnx porting: Use glnx_shutil_rm_rf_at()

libglnx porting: Use glnx_shutil_rm_rf_at()

In some cases (such as `ostree-sysroot-cleanup.c`), the surrounding
code would be substantially cleaner if it was also ported to
fd-relative, but I'm going to do that in a separate patch.

That way these patches are easier to review for mechanical
correctness.  I used an Emacs keyboard macro as the poor man's
[Coccinelle](http://coccinelle.lip6.fr/).