Recently we wanted to remove the Community Portal Solution from one of our Development instances.
Here, the solutions admin page doesn’t give the option of deleting or removing it.
To remove it, we need to uninstall all the solutions installed by the Dynamics 365 Portals – Community Portal.
Back in Dynamics 365 we can see so many solutions installed as part of community portal.
Here we need to remove the solutions in the reverse order of their installation. This information can be found in the solution entity within Dynamics 365 CE.
Thanks to this helpful post by Jon Birnbaum
Also, as soon as we start deleting the solutions, we’d get the Install option back again within the Solutions admin page for that instance.
Hope it helps..
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Nishant Rana's Weblog 
I’ve had to do this twice before – about 18 months ago, and 12 months ago. Neither time was a great experience. I managed to get down to the last 2 solutions, but couldn’t get any further as they seemed to both have dependencies on each other, and on the base solution, so I ended up having to ensure our solution we’d built was completely free of portal items, export and build into another vanilla system. Did you manage to get them all out?
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Yes Richard it worked for us. I can understand it is really painful to remove the solution one by one and eventually getting stuck because of dependencies.
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Instead of using the Web API with FetchXml you can also navigate to the area Settings > Solution History in the web frontend (classic UI) to view the order in which the solutions have been installed.
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