Indeed, in this specific case dropping the *pas* is the normal formulation. It's not impossible that the word may be restored to the expression in the future, but it is not a current usage at all.

Many verbs have or still show hesitation as to the proper way to construct them (just like some do in in English). In fact *rêver* **still** shows hesitation as to whether the correct preposition is *à* or *de*! It was once more common for that verb to also be constructed as a direct transitive verb, especially in the meaning of "meditate, think about, imagine, desire", which this expression may well have been derived from (Grévisse, *Le Bon usage 14e ed.*, §296 F). Today the usage is at best highly literary anywhere else but in this expression.

It is frequent for specific expressions (in all languages) to preserve otherwise obsolete fragments of syntax, grammar and meaning (English example include *fight with*, where *with* originally meant "against" and *hold fast* where *fast* means "firm, secure" *and* is an adverb without *-ly*).