When querying a realtime Continuous Aggregate using window functions the
new planner optimization to constify the `cagg_watermark` function call
is not working because we're checking for window function usage on the
query tree but actually this is a current limitation when creating a new
Continuous Aggregate, but users can use window functions when querying
it.
Fixed it by removing the check for `query->hasWindowFuncs` to prevent
the process of constification of the `cagg_watermak` function call.
Fixes#6722
