Remove legacy handling for 'fixing' line_item.entity_id

Remove legacy handling for 'fixing' line_item.entity_id

I tried to see what was happening with the test in https://github.com/civicrm/civicrm-core/pull/18113
and spotted that it wasn't using Order.api so the line items were likely wrong. However, once I set
it up to use the Order api I found the code we added back in 2018 to attempt to cope with other code
setting entity_id incorrectly was actually itself setting entity_id incorrectly. The issue
happens when there are 2 Memberships linked to one contribution & the removed code 'decides'
one is wrong and alters it. https://github.com/civicrm/civicrm-core/pull/11816

This line of code throws a deprecation notice - which has not been reported / apparently seen in the last
couple of years so I'm assuming it's not actually doing any good and without removing it this test actually
fails