Preston should be able to remove labels on update

[antoin-m]

For example :

1. I open a PR with a commit that modifies classic
2. Preston adds "Report on StarterTheme" label
3. I update the PR -> it doesn't change classic anymore
4. [IMPROVMENT] Preston should remove the "Report on StarterTheme" label

As suggested here : https://github.com/PrestaShop/prestonbot/issues/48

[mickaelandrieu]

Good idea, it will be done in 3.x branch

[mvorisek]

Same for version/branch label, see: https://github.com/PrestaShop/PrestaShop/pull/17288

originally 1.7.6.x label was added, then the base and the description was changed to 1.7.7.x branch. The new branch label was added successfully, but the original label remained.