Remove urllib3/requests from requirements

This patch removes urrlib3 and requests from the requirements.txt
file so that the CentOS Ceph gate will pass.

At the moment, yum tries to install python-urllib3 and throws errors
when it finds that urrlib3 files already exist (since they were
installed via pip).

This also breaks our periodic builds which, in turn, makes infra
quite displeased.

Change-Id: I85148daff49360980a58ff7b6a3e135214eb36fd
diff --git a/requirements.txt b/requirements.txt
index 00c7e64..8fe0322 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,7 +3,6 @@
 # process, which may cause wedges in the gate later.
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
-urllib3>=1.21.1 # MIT
 oslo.log>=3.30.0 # Apache-2.0
 oslo.config!=4.3.0,!=4.4.0,>=4.0.0 # Apache-2.0
 oslo.policy>=1.23.0 # Apache-2.0