networkRefreshDhcpDaemon: Get dnsmasq's PID once

networkRefreshDhcpDaemon: Get dnsmasq's PID once

This is a relic of commit v3.7.0-rc1~132 when getter/setter APIs
for dnsmasq's PID were introduced. Previously, obj->dnsmasqPid
was accessed directly. But the aforementioned commit introduced
two calls to virNetworkObjGetDnsmasqPid() even though the result
of the first call is stored in a variable.

Remove the second call as it's unnecessary.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>