Add removal of robot comments to Gerrit 3.13 release notes

This seems to have missed [0].

[0] https://github.com/GerritCodeReview/gerrit/commit/f890153eafbfd477530f4c5a9676783ed5582aa8

Change-Id: I7bfa303bb8ecaba43d3c8dd8f4d4e714ec9b85c8
2 files changed