Python: Update py/ineffectual-statement

e.(StrConst).isDocString() can only hold if e instanceof StrConst, since we have
that condition on the line above, we can safely remove this condition.

python/ql/src/Statements/StatementNoEffect.ql

@@ -125,7 +125,6 @@ predicate python2_print(Expr e) {
 
 predicate no_effect(Expr e) {
     not e instanceof StrConst and
-    not e.(StrConst).isDocString() and
     not e.hasSideEffects() and
     forall(Expr sub | sub = e.getASubExpression*() |
         not side_effecting_binary(sub) and