Update README.md in route53 plugin (#2299)

Update README.md in route53 plugin, as credentials_file is
not supported.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/route53/README.md

@@ -74,16 +74,6 @@ Enable route53 with fallthrough:
 }
 ~~~
 
-Enable route53 with AWS credentials file:
-
-~~~ txt
-. {
-    route53 example.org.:Z1Z2Z3Z4DZ5Z6Z7 {
-      credentials_file some-user
-    }
-}
-~~~
-
 Enable route53 with multiple hosted zones with the same domain:
 
 ~~~ txt