You can't "delete" from a text file - you have to create a new file, copy the data from the start up to the point of deletion, skip the deleted part, then copy the rest of the file up to the end. Then rename the original as .BAK or similar, and rename the copy to the original file name.
But...that looks like an INI file, and they are seriously outdated. Unless this is legacy data, you probably shouldn't be using INI format as there are a lot of better, more modern formats. If you must process them though, you can use
WritePrivateProfileStruct[
^] to delete sections and it will do all that for you.
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