Multisite: Revert [41698] and [41743].

In order for `get_site_by()` to be truly beneficial, caching in `WP_Site_Query` needs to be improved to account for common use-cases and have them be invalidated less aggressively.

See #40180, #40228, #42091.

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git-svn-id: https://core.svn.wordpress.org/trunk@41718 1a063a9b-81f0-0310-95a4-ce76da25c4cd