README: Remove issue-stats badges, the service is no longer available.

The issue-stats service is not well maintained and likely the situation
won't improve in the future.  See:
https://github.com/hstove/issue_stats/issues/41
https://github.com/hstove/issue_stats/issues/46

README.md

-[![Build Status][travis-img]][travis-repo] [![Coverage Status][coveralls-img]][coveralls-repo] [![Issue Stats][istats-pr-img]][istats-pr-repo] [![Issue Stats][istats-issue-img]][istats-issue-repo]
+[![Build Status][travis-img]][travis-repo] [![Coverage Status][coveralls-img]][coveralls-repo]
 [travis-img]:  https://travis-ci.org/micropython/micropython.png?branch=master
 [travis-repo]: https://travis-ci.org/micropython/micropython
 [coveralls-img]:  https://coveralls.io/repos/micropython/micropython/badge.png?branch=master
 [coveralls-repo]: https://coveralls.io/r/micropython/micropython?branch=master
-[istats-pr-img]: http://issuestats.com/github/micropython/micropython/badge/pr
-[istats-pr-repo]: http://issuestats.com/github/micropython/micropython
-[istats-issue-img]: http://issuestats.com/github/micropython/micropython/badge/issue
-[istats-issue-repo]: http://issuestats.com/github/micropython/micropython
 
 The MicroPython project
 =======================