unset mode query variable before checking base request

[alpipego]

Analogous to the paged parameter, unset mode whet checking whether
this is the base request for the \WP_Posts_List_Table.

Trac ticket: https://core.trac.wordpress.org/ticket/56599

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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