Fix another Assert issue exposed by CLOBBER_CACHE_ALWAYS.

Fix another Assert issue exposed by CLOBBER_CACHE_ALWAYS.

plpgsql's exec_stmt_execsql was Assert'ing that a CachedPlanSource was
is_valid immediately after exec_prepare_plan.  The risk factor in this case
is that after building the prepared statement, exec_prepare_plan calls
exec_simple_check_plan, which might try to generate a generic plan --- and
with CLOBBER_CACHE_ALWAYS or other unusual causes of invalidation, that
could result in an invalidation.  However, that path could only be taken
for a SELECT query, for which we need not set mod_stmt.  So in this case
I think it's best to just remove the Assert; it's okay to look at a
slightly-stale querytree for what we need here.  Per buildfarm testing.