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You cannot directly delete this item without a security message so extract the entire zip to a new unzipped Sitecore for Salesforce - Core-x.x folder then drill down and delete the WFFM item. Next, delete package.zip out of the original zip file and drag the entire modified package.zip across to the original. Install the modified zip as normal.
S4S Installation Chart
Please follow the steps in the S4S Installation Chart to save time and effort.
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