It’s only possible to remove plugins which you have added from a user folder. The pre-installed plugins use a system folder which is why the remove button is greyed out in the Plugin Manager.
For Smugmug you need the Publishing Manager. You can launch the dialog by clicking setup next to one on the unused services or by right-clicking on the SmugMug service and choosing edit.
For Some plugins, the “Remove” button is grayed out. All this means is that this plugin has been installed in a special place that Lightroom recognizes as being for plugins. Just like any other, you can find out exactly where it is by opening the “Status” panel. Because it was installed into this special location, Lightroom loads it automatically without needing to be told to “Add” it. Because you didn’t “Add” it, you also can’t “Remove” it.
Please let us know if it helps.