If you would rather not delete an unwanted question from a quiz, there are other options depending on how you want learners' grades to be affected.
Give learners extra credit
Make the unwanted question a bonus question and decrease the maximum points in the grade book. This option increases grades for all learners.
Give learners full credit
Give all learners full points for the unwanted question by using Update all attempts. The maximum points for the quiz remains the same. This option increases the grades for learners who got the question wrong, but the grades of learners who got it right does not change.
Do not give learners any credit
Change the unwanted question to a bonus question worth 0.00001 points (it is not possible for a question to be worth zero points), give all learners zero points using Update all attempts, and decrease the maximum points in the grade book. This option increases the grades of learners who got the question wrong and decreases the grades of learners who got it right (unless their score was 100%).