Removes paramiko dependency from test-requires

Removing the paramiko dependency from tools/test-requires because it already
exists in tools/pip-requires. The tox.ini file includes both pip-requires and
test-requires and tox aborts because the dependency is essentially specified twice
(even though they do not have contradictory version requirements)

Change-Id: Ifc68f9ee52ed9b0dffc73c9cb710f611b3471051