Remove duplicated requirement in tox file

Author: browniebroke

tox.ini

@@ -48,7 +48,6 @@ deps =
     drf314: djangorestframework>=3.14,<3.15
     pytest: -rrequirements.txt
     mypy: git+https://github.com/typeddjango/djangorestframework-stubs.git@946c7d60aaecdc9ef307f5e1f8eb55f7083ffb16#egg=djangorestframework-stubs
-    mypy: djangorestframework-stubs
 commands=
     pytest: pytest --cov drf_writable_nested --cov-report=xml
     mypy: mypy example