sidebar: Don't try to invoke parent's popup menu
This fixes a crash when hitting the menu key and displays the menu as expected. Otherwise it crashes like
(gdb) bt
#0 0x0000000000000000 in ()
#1 0x0000556b8bfacc93 in seahorse_sidebar_real_popup_menu (base=0x556b8c768240 [SeahorseSidebar]) at ../src/sidebar.vala:203
#4 0x00007f2a045594a9 in <emit signal ??? on instance 0x556b8c768240 [SeahorseSidebar]>
(node=<optimized out>, detail=0, instance=0x556b8c768240, emission_return=0x7ffc1b838a20, instance_and_params=0x556b8cc715c0) at ../../../gobject/gsignal.c:3781
#2 0x00007f2a03f11fd2 in _gtk_marshal_BOOLEAN__VOID
(closure=0x556b8c3c7e50, return_value=0x7ffc1b838930, n_param_values=<optimized out>, param_values=<optimized out>, invocation_hint=<optimized out>, marshal_data=<optimized out>) at gtkmarshalers.c:2110
#3 0x00007f2a045476cf in g_closure_invoke
(closure=closure@entry=0x556b8c3c7e50, return_value=return_value@entry=0x7ffc1b838930, n_param_values=1, param_values=param_values@entry=0x556b8cc715c0, invocation_hint=invocation_hint@entry=0x7ffc1b838910) at ../../../gobject/gclosure.c:830
#5 0x00007f2a03c4546f in gtk_binding_entry_activate (entry=0x556b8c3a4bc0, object=object@entry=0x556b8c768240 [SeahorseSidebar]) at ../../../../gtk/gtkbindings.c:646
#6 0x00007f2a03c45928 in binding_activate