Move sphinxcontrib-httpdomain to test-requirements

The sphinxcontrib-httpdomain is now in the global-requirements list, so,
it'll be better to track *all* requirements in the corresponding files.

Change-Id: I6cd9eb8982cd49be1c59680a16d66acd30b9a29f
Merge "Move sphinxcontrib-httpdomain to test-requirements"