How to remove all cell formatting
If you just want to remove hyperlinks, this post (the first result of this Google search) should help. Summarized below:


Select all cells using ⌘+A (or Edit -> Select All) 
Copy cells using ⌘+C ( or Edit -> Copy)
Paste and Match Style using ⌘+⌥+ ⇧ +V (or Edit -> Paste and Match Style)


This will remove all hyperlinks, however it will keep formatting like Bold, Italics, etc. If you want all formatting to be gone, use the suggestion from Steve Chambers to export to CSV and import from CSV, as the CSV Format is just plain text and doesn't save any formatting.


File -> Export to -> CSV...
Select the CSV you just exported in Finder and right click -> Open With -> Numbers


This will completely open a new Numbers document without formatting, unless you have Numbers set to automatically detect hyperlinks. If you do not want hyperlinks to be detected:


Open Numbers preferences using ⌘+, (or Numbers -> Preferences...)
Go to the Auto-Correction tab and uncheck "Automatically detect links" under the Formatting section