Revert "Add Pillow to test-requirements.txt"

This reverts commit db322f2fcadcb1b9a9ac32892d49b02b5df78cd2.

Adding Pillow to *requirements.txt was a temporary way to set the
upper bound to < 3.0.0

nodepool thick slaves and requirements have been fixed and devstack is
fixed in the patch that this depends on.

Change-Id: Ifee1356f2539c8fa0ef51fb9c43c0d9c07ca4f86
Depends-On: I2bec8cf1bfeaaa6ae329704229fdeb86d26e55c7
1 file changed