Dropping automatic tests for Debian 7 Wheezy.

Debian 7 Wheezy has been superseded by Debian 8 Jessie on Apr 25, 2015,
with LTS support ending on May 31, 2018.

Debian 7 Wheezy's default GCC doesn't fully support C11. It should
anyway still be possible to build BIRD for Debian 7 if you backport
a C11-capable compiler there.