Remove assertion checking query ID in execMain.c

Remove assertion checking query ID in execMain.c

This assertion has been added by 24f520594809, but Alexander Lakhin has
proved that the ExecutorRun() one can be broken by using a PL function
that manipulates compute_query_id and track_activities, while the ones
in ExecutorFinish() and ExecutorEnd() could be triggered when cleaning
up portals at the beginning of a new query execution.

Discussion: https://postgr.es/m/b37d8e6c-e83d-e157-8865-1b2460a6aef2@gmail.com