To build on @JeffDUnity3D 's advice with these upgrade instructions A combined com.unity.purchasing package, merges the Unity IAP plugin: 3.0.0 - performing Reimport All on the project should erase the Library/ folder.
Seeing Library\ScriptAssemblies\Purchasing.Common.dll means this DLL file is being used by Unity, and that DLL file will be deleted when Assets/Plugins/UnityPurchasing is deleted. Unity should automatically import the project, and remove the files which are no longer located under Assets/Plugins/UnityPurchasing. However, if Unity does not automatically delete, then Reimport All should rebuild the Library folder, based upon what is inside the project’s Assets/ and Packages.