Remove obsolete ./configure option

The '--enable-vty-tests' is just alias to '--enable-external-tests'
anyway.

Change-Id: I8ab3ac3d9becee6b4e989b4e71d34598ea012f47
diff --git a/README.vty-tests b/README.vty-tests
index ba1b87c..0669ea8 100644
--- a/README.vty-tests
+++ b/README.vty-tests
@@ -4,7 +4,7 @@
 
 and pass the following configure options here:
 
-  ./configure --enable-vty-tests --enable-external-tests
+  ./configure --enable-external-tests
 
 The VTY tests are then included in the standard check target: