xap_helper.h: remove _SC_NPROCESSORS_ONLN default
It's never straightforward to pick an ideal number of processes
for anything and Xapian helper processes are no exception since
there may be a massive disparities in CPU count and I/O
performance. So default to a single worker for now in the C++
version since that's the default is for the Perl/(XS|SWIG)
version, and also the same as for our normal public-facing
daemons.
This keeps the behavior between the Perl+(XS|SWIG) and C++
version as similar as possible.