Removing a workflow is a two fold thing means if you just want to make the workflow unavailable to users or actually deleting it.
Removing a workflow through a browser does not actually delete it.
If a workflow is created through SharePoint Designer and removed through a browser, it just gets unavailable to users but all the source files used to compile a workflow remains stored in Workflow Document Library of the site.
To remove a workflow, follow these steps
1. Browser to the list which you want the workflow to be removed from.
2. On the list library menu bar, click Settings and then click List Settings
3. You will end up on page where you can customize your list. Under Permissions and Management, click Workflow Settings
Note: If you have a workflow associated with this list, you will see the list of workflows added and will be given an option to remove or add another workflow but if there is no workflow attached, you will be directed to "Add a workflow" page from where you can create Out of the Box workflows.
4. If you have workflows attached to your list, you are on Change Workflow Settings Page, click Remove a workflow hyperlink.
5. You will be provided with a list of workflows that are associated with the list. Click Remove radio button against the workflow that you want removed.
6. Click OK
Note: There is a difference between No New Instance and Remove. If you remove a workflow from a list by selecting Remove option, it will cancel all the running instances of the workflow. If you want to keep the current instance running, select No New Instance. This will help you complete the already instance and in the meantime will not allow any new action to be performed from the workflow that you want removed. Once all the instances are complete, you can come back to this screen and remove the workflow.
To delete a workflow, follow these steps
Deleting a workflow from SharePoint Designer is simple and as discussed earlier, deleting a workflow from SharePoint Designer will delete all the source files from the site and will stop all the workflow instances that are currently running on the list or library.
1. Open your site in SharePoint Designer.
2. From the Folder List Pane on the left of your SharePoint Designer, expand Workflows Folder
Note: If Folder List is not visible, click View>Folder List to make it visible.
3. Select the workflow that you want to delete and press Delete button on the keyboard. A confirmation message will be shown to you to confirm if you actually want to delete it, click Yes
Note: You can select multiple workflows by clicking workflows and holding CTRL key.