Deprecate sensitivity list on public_flat_rw attributes (#6443)

These are no longer required for correct scheduling. They are still
accepted for backward compatibility, but have no effect on simulation
and are dropped in the front-end. Also removed the then redundant
AstAlwaysPublic class.

Fixes #6442

src/V3OrderGraphBuilder.cpp

@@ -334,9 +334,6 @@ class OrderGraphBuilder final : public VNVisitor {
     void visit(AstAssignW* nodep) override { iterateLogic(nodep); }
 
     //--- Verilator concoctions
-    void visit(AstAlwaysPublic* nodep) override {  //
-        iterateLogic(nodep);
-    }
     void visit(AstCoverToggle* nodep) override {  //
         iterateLogic(nodep);
     }