BUG #19073: there are meaningless code in _SPI_execute_plan() when canSetTag is true

The following bug has been logged on the website:

Bug reference:      19073
Logged by:          Webbo Han
Email address:      webbohan@qq.com
PostgreSQL version: 18.0
Operating system:   all
Description:

In function _SPI_execute_plan(), `my_tuptable` is local variable and
initialized as NULL.
If plan is canSetTag subqueries, `SPI_freetuptable(my_tuptable)` is
meaningless, because `my_tuptable` is NULL.
We'd better remove this code.