Is it possible to clear the "abuse or spam" flag after the offending answer was deleted?
As far as I can tell, no.
The question you use as an example is only listed as being flagged for abuse/spam in the Questions
I've come across this issue before, i.e. questions that are still listed as being flagged for abuse/spam after the answer was removed, and I can't say that I have a solution to this bug.
I used the question you posted here as an example to try and find a fix.
I've tried reposting an answer, flagging it for abuse/spam, then removing it, but the question is still being listed as flagged in the Answers that need help list.
I've tried posting another answer, flagging it for abuse/spam, updating it to clear the flag, then removing the answer, but the outcome was the same.
I've moved this question to the Bug Reports topic because this is clearly a bug of some sort. Luckly, this does not happen for most questions for which spam/abuse answers are removed.
So, the bottom line is that I don't think that we can get rid of the flag after the answer was removed. If we remove an answer and the flag is still there, then we're definitely in bug territory.