Drop duplicate tox config

"usedevelop" is specified and set to true
in the default testenv and doesn't need to be
specified in the specific environments.

Change-Id: I8adee2feec3ac46f22c04cae86ed0559785ee414

tox.ini

@@ -32,7 +32,6 @@ commands =
 commands = {posargs}
 
 [testenv:docs]
-usedevelop = True
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
@@ -42,7 +41,6 @@ commands =
   sphinx-build -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
-usedevelop = True
 deps = {[testenv:docs]deps}
 allowlist_externals =
   make