How to delete import presets?

The Delete option presented in the same place where you would select which preset to use, is a widespread method used across many LrC features. That works one at a time.

Another method lets you go to where these presets are stored on disk: where you can delete those, copy them to/from another computer, back them up and so forth. Go to Lightroom Preferences in the menu; Presets tab; Location section. There is one button to show your saved Develop presets (these are shared with Adobe Camera Raw) and another to Show All Other (these are not). The second button opens a file browser window with a "Lightroom" folder selected. Inside that are the subfolders for various kinds of presets including your import presets, export presets ... etc 

If you delete or add something here, no change will be seen within the Catalog until you've closed and reopened that. I believe (the organisation of all this has changed around quite a bit over time) if you've created a preset yourself, the proper place for that may be the respective User Presets subfolder, not the Lightroom Presets subfolder alongside. It certainly used to be the case that only a User Presets location would get re-scanned for any changes whenever LrC started up.

You need to select the import preset that you want to delete, then choose the delete option. See attached screenshot for Import Presets menu options.