regexp: remove unnecessary sentence in doc comment.

R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/53190046

src/pkg/regexp/regexp.go

@@ -70,7 +70,6 @@ import (
 var debug = false
 
 // Regexp is the representation of a compiled regular expression.
-// The public interface is entirely through methods.
 // A Regexp is safe for concurrent use by multiple goroutines.
 type Regexp struct {
 	// read-only after Compile