Use this field in place of the simple { PAGE } field:

{ IF { PAGE } < { NUMPAGES } { PAGE } }

This will insert the page number on all but the last page. Note that each
pair of field delimiters (the things that look like braces but can't be
typed from the keyboard) must be inserted using Ctrl+F9, but if you insert a
pair for the IF field, you can use the Insert Page Number and Insert Number
of Pages buttons on the Header and Footer toolbar to insert the remaining
fields.