Drop fossilized '[testenv:py26]' tox environment.

We are now 2.6-compatible in the default environment.

Author: tseaver

tox.ini

@@ -2,14 +2,6 @@
 envlist =
     py26,py27,py33,py34,cover,docs,lint
 
-[testenv:py26]
-commands =
-    nosetests
-deps =
-    nose
-    unittest2
-    protobuf==3.0.0-alpha-1
-
 [testenv]
 install_command =
     {toxinidir}/scripts/custom_pip_install.sh {opts} {packages}