Remove extra definition of je_tsd_boot on win32.

Looks like `je_tsd_boot` gets defined twice when building on Windows. This causes a [C2084 build error](https://treeherder.mozilla.org/ui/logviewer.html#?job_id=3229575&repo=try) whenever `tsd.h` gets included. I have tried to fix this by removing the trivial implementation of `je_tsd_boot`, since the other seems to be the correct one.