Remove docs/requirements.txt which should no longer be used

Slowly trying to reduce the number of places we have dependencies specified. The documentation building processes for both `tox -e docs` and readthedocs.org should now use the dependencies specified in `setup.py`.

Author: kotfu

docs/requirements.txt

@@ -1,5 +1,5 @@
 [tox]
-envlist = py34,py35,py36,py37,py35-win,py36-win,py37-win
+envlist = docs,py34,py35,py36,py37,py35-win,py36-win,py37-win
 
 [pytest]
 testpaths = tests
@@ -11,9 +11,11 @@ setenv =
 
 [testenv:docs]
 basepython = python3.7
+deps =
+  sphinx
+  sphinx-rtd-theme
 changedir = docs
-extras = dev
-commands = sphinx-build -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
+commands = sphinx-build -a -b html -d {envtmpdir}/doctrees . {envtmpdir}/html
 
 [testenv:py34]
 deps =