Remove tempest-lib from requirements.txt

It is not used and also deprecated in favor of tempest.lib (from tempest
itself).

Change-Id: I18f5c726513b68083d8a5793b8eb391b421b8514
diff --git a/requirements.txt b/requirements.txt
index 415ded0..1afcc6f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,5 +5,4 @@
 pbr>=1.8 # Apache-2.0
 
 tempest>=12.1.0  # Apache-2.0
-tempest-lib>=0.14.0  # Apache-2.0
 cryptography!=1.3.0,>=1.0 # BSD/Apache-2.0