In Adwaita there is no linked style for GtkFrame.
<object class="GtkBox">
<child><object class="GtkButton"></object></child>
<child><object class="GtkFrame">
<property name="shadow_type">in</property>
</object></child>
<child><object class="GtkButton"></object></child>
<style>
<class name="linked"/>
</style>
</object>
and with css rule
.custom-frame border {
border-right-width: 0px;
}