[Backport] Remove routes, action links and help text related to installing new code via the Drupal UI

Closes #3417136

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• 6ae683da - [#3417136] Remove tests of the install feature that's now hidden

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added 4 commits

• 7ce19f07 - [#3417136] Remove mention of "add" modules in system_help, update docs for system_authorized_init()
• 163af809 - [#3417136] Remove stale docs about Updater classes
• 5c17f533 - [#3417136] Trigger deprecation in update_authorize_run_install()
• e94b9a2d - [#3417136] Trigger deprecation in UpdateManagerInstall form

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added 1 commit

• 49d9f32d - [#3417136] Fix syntax error for deprecation in update.authorize.inc

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closed