Remove release tags

gym-unity/gym_unity/__init__.py

 __version__ = "0.26.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_15"
+__release_tag__ = None

ml-agents-envs/mlagents_envs/__init__.py

 __version__ = "0.26.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_15"
+__release_tag__ = None

ml-agents/mlagents/trainers/__init__.py

 __version__ = "0.26.0.dev0"
 
 # Git tag that will be checked to determine whether to trigger upload to pypi
-__release_tag__ = "release_15"
+__release_tag__ = None