Nowdays, Primefaces 5.x have a attribute in panelGrid named "columnClasses".
.no-border {
border-style: hidden !important ; /* or none */
}
So, to a panelGrid with 2 columns, repeat two times the css class.
<p:panelGrid columns="2" columnClasses="no-border, no-border">
To other elements, the ugly " !important " is not necessary, but to the border just with it work fine to me.