[readme] remove long-dead testling-ci badge

Author: ljharb

readme.markdown

@@ -2,8 +2,6 @@
 
 tap-producing test harness for node and browsers
 
-[![browser support](https://ci.testling.com/substack/tape.png)](http://ci.testling.com/substack/tape)
-
 [![build status](https://secure.travis-ci.org/substack/tape.svg?branch=master)](http://travis-ci.org/substack/tape)
 
 ![tape](https://web.archive.org/web/20170612184731if_/http://substack.net/images/tape_drive.png)