How can I dynamically remove an entity constraint?
Validation constraints are plugins with an alter hook: hook_validation_constraint_alter() (the hook name can be seen in \Drupal\Core\Validation\ConstraintManager::__construct()).

That means you can alter that plugin and replace it with your own class that doesn't do anything, or only under some conditions.

Note that constraint plugins are special, unlike everything else, they consist of two classes, the plugin class is just the definition and strings, and then here is a $classname . 'Validator' class that contains the actual logic. So you need two classes, first the constraint class, Drupal\yourmodule\Something and then an overriden version of the Validator as Drupal\yourmodule\SomethingValidator.