Issue #2163035 by Berdir: Remove $user->theme and Drupal\user\Theme\UserNegotiator.

...ext/lib/Drupal/text/Tests/Formatter/TextPlainUnitTest.php

core/modules/user/lib/Drupal/user/Entity/User.php

...dules/user/lib/Drupal/user/Tests/UserRegistrationTest.php

...modules/user/lib/Drupal/user/Tests/UserValidationTest.php

core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php

core/modules/user/lib/Drupal/user/UserInterface.php

core/modules/user/user.install

core/modules/user/user.services.yml

...views/lib/Drupal/views/Tests/Handler/FieldCounterTest.php

...es/views/lib/Drupal/views/Tests/Handler/FieldUnitTest.php

...s/views/lib/Drupal/views/Tests/Plugin/DisplayPageTest.php