How do you delete an icon from Launchpad that is for a software you deleted from your iMac?

Try clicking and holding the app. When you do this, as with an iPhone they will jiggle so you can move them. In this mode, does the app have a small "X" next to it? If so, then you can click this to remove it.

Hold down the option key an click icon in the Launchpad. Icons will jiggle and an X will appear.

Click the X . Click elsewhere to stop jiggling.