Currently PRODUCTEXTENSION does not have a value, which is unfortunate
because some places such as the soffice --help output and
utl::DocInfoHelper::GetGeneratorString() which creates the
meta:generator for ODF files (via Setup.xcu Product/ooSetupExtension)
write an incomplete version number.

Since there doesn't appear to be a point in having 2 variables here,
remove SHORT_ one and use PRODUCTEXTENSION consistently.

Change-Id: I5aac25bcedd785a098cba612643ea68fe19c77f0