Use PG_FINALLY to simplify code

Hi

In (/src/pl/plpgsql/src/pl_exec.c), I found some code like the following:

     PG_CATCH();
    {
        if (expr->plan && !expr->plan->saved)
            expr->plan = NULL;
        PG_RE_THROW();
    }
    PG_END_TRY();

    if (expr->plan && !expr->plan->saved)
        expr->plan = NULL;

Since PG_FINALLY can be used now, I think we can use PG_FINALLY to simplify code here.

Best regards,
houzj