The MSstats external tool uses a report in Skyline called "MSstats Input".
You can get to that report using the Document Grid. There is a "Reports" dropdown at the top of the Document Grid, and you can choose:
External Tools > MSstats Input
After you have selected the MSstats Input report, you can choose "Edit Report" from the Reports dropdown.
On the "Filter" tab of the Customize Report dialog, you can add a filter for the "Fragment Ion" column.
In the attached screenshot, I have added the "Does not start with...precursor" filter.
Once you have made this change to the MSstats Input report, it will affect the data that the MSstats external tool sees.
This is probably easier than actually deleting all of the precursors from your Skyline document.
If you later want to use MSstats with precursor data, you will need to change the report back to the way it was.
If you want to learn more about the Document Grid and reports, you should take a look at the Custom Reports tutorial: