net-mgmt/mbrowse: reduce overlinking and remove pkg dep

First no ports should ever be depending on pkg directly because it
prevents building with WITH_PKG=devel, they should depend on
${PKG_ORIGIN}.
Second, netsnmp do not expose the pkg API but only consume it, so
program linking to netsnmp should not end up being directly linked to
libpkg!

Patch the configure script to stop asking net-snmp-config for extra libs
which will faultly bring in linking against -lpkg and -lelf, and remove
entirely libpkg as a dependency on the package