The removal of third-party input methods or apps is different from regular apps. You can’t find input method apps from the Applications folder in Finder or drag them to the trash can from there. Input Methods are system level plug-ins. They are saved to the “/Library/Input Methods” or “/Library/Components” folder with system administrator privilege. So if you decide not to use certain input method anymore, find and run Activity Monitor app from the Launchpad, find and force stop the associated service running in the background. Then open Finder, click Go from the top menubar, choose Library from the drop-down list to open it in Finder. Find and open the Input Methods folder. Then select and move the input method to trash to uninstall it.
In fact, the developers may provide the uninstaller. Download the input method app or installer, double click to run it see if you can find the uninstallation tool. In this way, the app logs, caches would usually be cleaned automatically with no leftovers.
Other than these two methods, you can also download and install a third-party input method removal app which usually allow users to easily check the Input Methods they decide not to use anymore and one-click to quickly uninstall them.