Remove epel8 from platform stream expansion

None of the nextcloud module streams in epel8 are functional due to
missing runtime dependencies.  See rhbz#2042653 for more information.

nextcloud.yaml

@@ -15,9 +15,9 @@ data:
             - MIT
     dependencies:
       - buildrequires:
-            platform: [-eln]
+            platform: [-eln, -epel8]
         requires:
-            platform: [-eln]
+            platform: [-eln, -epel8]
     references:
         documentation: https://docs.nextcloud.com/server/latest/admin_manual/
         tracker: https://github.com/nextcloud/server/issues