topic: drop the old way to handle topic changes on amends
This seems to be a very old way of making sure topics can be cleared by amending. See 565f057bdc08 and ec28b6c3414a, both of which are from 2015 (around the time of hg 3.6).
For the current take on handling amending in topic extension see 6275808e89ef.
Edited by Raphaël Gomès