l10n: Avoid calling is_user_logged_in() in determine_locale().

is_user_logged_in() is a pluggable function, and loaded after plugins are loaded. If a plugin calls __() too early, is_user_logged_in() is missing and WordPress will fatal. get_user_locale() already handles this scenario for us, so it's safe to rely on exclusively.

Props danielbachhuber.
Merges [43846] to trunk.
See #44758.
Fixes #45235.

Built from https://develop.svn.wordpress.org/trunk@44181


git-svn-id: https://core.svn.wordpress.org/trunk@44011 1a063a9b-81f0-0310-95a4-ce76da25c4cd