For people coming to this page from internet searches (as I did): as of October 2023 (not sure what version of Canvas), in New Quizzes it is possible to adjust point values for questions (or to assign zero points to a question that was mistakenly posed) after students have already taken a quiz. Just follow the instructions for Regrading a New Quiz. However, that page notes that "Regrading is not available for Essay or File Upload questions," so it seems the OP is still out of luck. Note that although you initially adjust the score on a single student's quiz in Speedgrader (and not in the Build section of the New Quiz), the Regrade function can update the point value for all students.
I needed to exclude some invalid questions from a New Quiz students had already taken. In order to do that, you can assign zero points to the bad question(s) in Speedgrader and Regrade, choosing the option to give students full credit for the question regardless of their answer. Canvas will recalculate both the individual student grades and the total points for the quiz (the total points assigned to the quiz in Assignments and Gradebook remains the same, but the individual student scores are properly adjusted based on the percentage score of the regraded quiz).
Incidentally, this was useful to me to exclude some questions whose uploaded images didn't render in Lockdown Browser when those questions were imported from a Classic Quiz to a New Quiz using the Migrate function. Tech support confirmed this is currently a bug in Lockdown Browser and/or Canvas when using the Migrate function on Classic Quizzes with rich content (i.e., image files). A workaround is to put all the questions from a Classic question bank into a Classic Quiz, export and download that quiz as a QTI file, then create a New Quiz by uploading and importing the QTI file. The imported images will then correctly render in Lockdown Browser. Hopefully the Migrate bug is fixed soon.