Removing commented dead code without removing the legitimate comments
echo | grep is unwarranted. bash understands regular expressions ("$line" =~ regex), and can do simple substitutions: line=${line#[[:space:]]*\/\/} removes leading whitespaces , followed by the comment, just what we are after. 
Replacing the file while reading it looks suspiciously. I recommend to have a destination file, and copy valid lines (and don't copy undesired ones). A perk benefit is that forking sed is not needed anymore.


A side note: the script makes a false positive in cases like

    // Notice that
    // some_valid_c_code;
    // doesn't work, because etc


The part of the comment would be recognized as a dead code, and the output will be

    // Notice that
    // doesn't work, because etc