Fix unable to remove best answer

[dsevillamartin]

Fix unable to remove best answer, added logic for ID being 0. Not sure why this wasn't here. 🤔

Simplified some of the if logic as well so it wouldn't need be repeated.

[dsevillamartin]

@wiwatsrt If you aren't able to maintain this extension, we can move it into FriendsOfFlarum.

[dsevillamartin]

@wiwatsrt Offer still stands btw.

[dsevillamartin]

We've updated this extension under our FriendsOfFlarum namespace, see https://github.com/FriendsOfFlarum/best-answer.