It is unused in LO source code.

It was not really clearly defined what it meant and what it could be used
for. Its value was ".exe" for Windows and ".bin" for other platforms. But we
do have a "soffice.bin" on Windows, too, corresponding exactly to the
"soffice.bin" on Linux. On the other hand, on OS X we don't have any
"soffice.bin", just a "soffice".

Change-Id: Iceee19cc8768c5aaaeeb9bba34b9cde50073498b