Do not set spiller to empty in SpillableHashAggregationBuilder

[sopel39]

Setting spiller to empty (without closing) caused spill files leak. Spiller is closed when SpillableHashAggregationBuilder is closed.

Description

Additional context and related issues

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## General
* Fix spill files leaking in aggregation queries. ({issue}`issuenumber`)

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Karol Sobczak.
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[sopel39]

cc @osscm