Refactor some codes about guc gp_eager_two_phase_agg

[charliettxx]

In previous PR, we introduced GUC gp_eager_two_phase_agg to force planner generating multi-stage agg path in Planner, but applying disable costs may not a good choice, we should force planner generate only multi-stage agg paths instead of single-stage agg paths. So after this patch, single-stage agg path will not exist in our planner path.

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