You have three cases:

• Line start with --, you want to remove the entire line aka replace with nothing.
• A substring starting and ending with " should be left alone, aka replace with itself.
• -- elsewhere should have the part after the -- removed, aka, replace the entire thing with --.

This is trivial in a regexp:

s/("[^"]*")|^--.*\n|(--).*/$1||$2||""/egm;
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