src/libpcp_web/src/exports: remove duplicate pmDiscoverClose entry
src/libpcp_web/src/exports: remove duplicate pmDiscoverClose entry

Ignored in most places, but was causing a build error on OpenIndiana
2019.04.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp