Simplify the WITHOUT ROWID code a bit

In the Table class we need to store whether this is a WITHOUT ROWID
table or now. Instead of just storing a boolean flag for that we were
storing a list of the rowid column(s). This is not just more complicated
to handle than a simple flag but also more error-prone because the list
must always be kept equal to the list of primary key columns. Failing to
keep them equal would result in an invalid SQL statement.

src/tests/testsqlobjects.cpp

@@ -124,7 +124,7 @@ void TestTable::withoutRowid()
     f.setAutoIncrement(true);
     tt.fields.push_back(f);
     tt.fields.emplace_back("b", "integer");
-    tt.setRowidColumns({"a"});
+    tt.setWithoutRowidTable(true);
     tt.addConstraint({f.name()}, ConstraintPtr(new PrimaryKeyConstraint()));
 
     QCOMPARE(tt.sql(), QString("CREATE TABLE \"testtable\" (\n"