Remove lower call in async_reserve (#143682)

async_reserve is only called from the the entity_platform helper
which already ensures the entity_id is validated and in lower
case.
https://github.com/home-assistant/core/blob/a783b6a0abda02b26e193356c4f3db8b86e13b86/homeassistant/helpers/entity_platform.py#L936

homeassistant/core.py

@@ -2234,7 +2234,6 @@ class StateMachine:
         This avoids a race condition where multiple entities with the same
         entity_id are added.
         """
-        entity_id = entity_id.lower()
         if entity_id in self._states_data or entity_id in self._reservations:
             raise HomeAssistantError(
                 "async_reserve must not be called once the state is in the state"