common: improve documentation comments (#16701)

This commit adds many comments and removes unused code.
It also removes the EmptyHash function, which had some uses
but was silly.

core/state/state_test.go

@@ -99,7 +99,7 @@ func (s *StateSuite) TestNull(c *checker.C) {
 	s.state.SetState(address, common.Hash{}, value)
 	s.state.Commit(false)
 	value = s.state.GetState(address, common.Hash{})
-	if !common.EmptyHash(value) {
+	if value != (common.Hash{}) {
 		c.Errorf("expected empty hash. got %x", value)
 	}
 }