Unused variables in images/kernel_read_write

There are many unused variables in images/kernel_read_write and it is not clear whether these are truly unneeded or whether they reveal a hole in the testing logic.

For example most test_read_*.cpp files contain `float maxErr = get_max_relative_error(...)` which is later hardcoded to `0.5`.

Start by reviewing the files under images/kernel_read_write, especially the test_read_*.cpp files, and trace how values such as maxErr from get_max_relative_error are used. Determine whether each unused variable reflects dead code or a testing gap; done means the variables and tests consistently represent the intended checks, with the relevant test suite passing.

Written by the indexing model from the issue text.