rego: Fix godoc for rego.Rego() API

The documentation was updated part way through adding the Prepare
helpers on the Rego object. Later on when PrepareForEval and
PrepareForPartial helpers were added and we stopped re-using
PartialResult this was left behind and is now incorrect.

Fixes: open-policy-agent#1619
Signed-off-by: Patrick East <east.patrick@gmail.com>

rego/rego.go

@@ -49,9 +49,6 @@ type PartialResult struct {
 }
 
 // Rego returns an object that can be evaluated to produce a query result.
-// If rego.Rego#Prepare was used to create the PartialResult this may lose
-// the pre-parsed/compiled parts of the original Rego object. In those cases
-// using rego.PartialResult#Eval is likely to be more performant.
 func (pr PartialResult) Rego(options ...func(*Rego)) *Rego {
 	options = append(options, Compiler(pr.compiler), Store(pr.store), ParsedQuery(pr.body))
 	return New(options...)