index_build does not need its isprimary argument

Hi all,

While working on REINDEX CONCURRENTLY, I have noticed that
index_build() does not need its argument isprimary.  Perhaps it is
not worth bothering, but for the REINDEX CONCURRENTLY business this
removes the need to open an index when triggering a concurrent
build.

The flag was introduced in 3fdeb189, but f66e8bf actually forgot to
finish the cleanup as index_update_stats() has simplified its
interface.

Are there any objections if I cleanup that stuff as per the attached?

Thanks,
--
Michael