chore: remove mypy-constrains reference as we don't need it anymore

Signed-off-by: Yurii Serhiichuk <[email protected]>

Author: xSAVIKx

tox.ini

@@ -42,7 +42,6 @@ commands = mypy cloudevents
 [testenv:mypy-samples-{image,json}]
 basepython = python3.11
 setenv =
-    PIP_CONSTRAINT={toxinidir}/requirements/mypy-constraints.txt
     mypy-samples-image: SAMPLE_DIR={toxinidir}/samples/http-image-cloudevents
     mypy-samples-json: SAMPLE_DIR={toxinidir}/samples/http-json-cloudevents
 deps =