Remove unsupported qiskit-ibmq-provider from docs job

The tox docs job definition was previously installing the
qiskit-ibmq-provider which is no longer supported and the project is
orphaned. This commit fixes this by removing the unnecessary dependency.

tox.ini

@@ -51,7 +51,6 @@ commands = black qiskit_aer test tools setup.py
 deps =
   -r requirements-dev.txt
   build
-  qiskit-ibmq-provider
 commands =
   python -I -m build --wheel -C=--build-option=-j4
   pip install --find-links={toxinidir}/dist qiskit_aer