releasetools: Remove a dead line.

system_root_image has been dead since commit
2ce63edab715ae4ff34e1a2ef240b05216f6ca5d.

Test: pylint --rcfile=pylintrc add_img_to_target_files.py
Change-Id: Id791747b7313923b82279a21d264a998455a92d4
1 file changed