Within the Rank Tracker Overview, you'll be able to see all the keywords that you are currently tracking for a project.

By selecting any keyword within the list via the checkbox, a "Delete" button will appear beside "Edit tags".
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So you can simply select the keywords you no longer require on Rank Tracker and hit "Delete" to remove those keywords.

If you wish to bulk delete keywords, you can do so by selecting all keywords (up to 50) on each page and then hitting "Delete" to remove those keywords.