Remove blurred image referenced in exercise 15

The repo has the blurred output file from the exercise, it should be generated when the code is built and run.

Locate the blurred output file referenced by exercise 15 and inspect how that exercise's code generates its output. Remove the checked-in blurred file while preserving the exercise materials; done means the file is absent and the output can still be generated by building and running the code.

Written by the indexing model from the issue text.