Remove unused test_requirements.txt

This file was only referenced in .travis.yml and that reference was
removed in 52258069b7058fa1800177c9f33d3c699a95ef79.

test_requirements.txt

@@ -1,3 +0,0 @@
-# Requirements used for testing
-mock >= 0.5.0
-meld3 >= 0.6.5