Remove INKSCAPE_PORTABLE_PROFILE_DIR in favour of INKSCAPE_PROFILE_DIR

As suggested in the title I'd like to drop the environment variable INKSCAPE_PORTABLE_PROFILE_DIR.

It was initially introduced for the "PortableApps" version of Inkscape (see https://bugs.launchpad.net/inkscape/+bug/498801) but has mostly been replaced by INKSCAPE_PROFILE_DIR which was implemented in https://bugs.launchpad.net/inkscape/+bug/1247448.

As db05b842 dropped a lot of platform-specific code (including one usage of INKSCAPE_PORTABLE_PROFILE_DIR) both variables serve identical functionality now. As I think we can do without the modifications to the registry (find some explanation for "App Paths" registry key here), which was the only remaining difference between them, there's no point in keeping both variables.

@GordCaswell Would it be fine with you if I removed that variable in master (no change in 0.92.x planned)? I guess adjusting the PortableApps version would be straightforward?
Does anybody know of any other usage of INKSCAPE_PORTABLE_PROFILE_DIR except for the "PortableApps" version?