Blender Git Loki

Preferences: remove temp directory initialization for WIN32

Revert 76b1a27f96ffe1ec8c5351f34bcc2b9733b4483e since there is no
reason windows should behave differently to other platforms.

This was added so Windows users wouldn't see "/tmp/" in the UI.

Since then the default temporary directory is a blank string,
leave blank on all systems as Python script authors may accidentally
use this instead of `bpy.app.tempdir`.