internal: inline CondVal function

Change-Id: Ic1115ab639e2d7b499c3400b5310575a36b1b796
Reviewed-on: https://go-review.googlesource.com/85320
Reviewed-by: Tim Cooper <tim.cooper@layeh.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/oauth2.go

@@ -35,10 +35,3 @@ func ParseKey(key []byte) (*rsa.PrivateKey, error) {
 	}
 	return parsed, nil
 }
-
-func CondVal(v string) []string {
-	if v == "" {
-		return nil
-	}
-	return []string{v}
-}