Remove deduplication logic from find_window_functions

Remove deduplication logic from find_window_functions

This code thought it was optimizing WindowAgg evaluation by getting rid
of duplicate WindowFuncs, but it turns out all it does today is lead to
cost-underestimations and makes it possible that optimize_window_clauses
could miss some of the WindowFuncs that must receive an updated winref.

The deduplication likely was useful when it was first added, but since
the projection code was changed in b8d7f053c, the list of WindowFuncs
gathered by find_window_functions isn't used during execution.  Instead,
the expression evaluation code will process the node's targetlist to find
the WindowFuncs.

The reason the deduplication could cause issues for
optimize_window_clauses() is because if a WindowFunc is moved to another
WindowClause, the winref is adjusted to reference the new WindowClause.
If any duplicate WindowFuncs were discarded in find_window_functions()
then the WindowFuncLists may not include all the WindowFuncs that need
their winref adjusted.  This could lead to an error message such as:

ERROR:  WindowFunc with winref 2 assigned to WindowAgg with winref 1

The back-branches will receive a different fix so that the WindowAgg costs
are not affected.

Author: Meng Zhang <mza117jc@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAErYLFAuxmW0UVdgrz7iiuNrxGQnFK_OP9hBD5CUzRgjrVrz=Q@mail.gmail.com