Remove driver references from QIR trees

src/qir/ast/operator/QIRGroup.java

src/qir/ast/operator/QIROperator.java

src/qir/ast/operator/QIROrder.java

src/qir/ast/operator/QIRProject.java

src/qir/ast/operator/QIRScan.java

src/qir/ast/operator/QIRSelect.java

src/qir/driver/PostgreSQLDriver.java

src/qir/factory/QIRFactory.java