Partially revert f0aa3f20f7

It seems that the Heroku build step has HEROKU_APP_ID but not all
the other environment variables.

>  AssertionError: For a production set up please set the missing variables: DATABASE_URL, TREEHERDER_DJANGO_SECRET_KEY

treeherder/config/settings.py

@@ -26,18 +26,6 @@ IS_WINDOWS = "windows" in platform.system().lower()
 # Top Level configuration
 DEBUG = env.bool("TREEHERDER_DEBUG", default=False)
 
-#  Default environment variables check
-REQUIRED_PRODUCTION_ENV_VARIABLES = ["BROKER_URL",
-                                     "DATABASE_URL",
-                                     "REDIS_URL",
-                                     "TREEHERDER_DJANGO_SECRET_KEY"]
-if os.environ.get("HEROKU_APP_ID"):
-    for env in REQUIRED_PRODUCTION_ENV_VARIABLES:
-        if os.environ.get(env):
-            REQUIRED_PRODUCTION_ENV_VARIABLES.remove(env)
-    assert len(REQUIRED_PRODUCTION_ENV_VARIABLES) == 0, "For a production set up please set the missing variables: {}".format(", ".join(REQUIRED_PRODUCTION_ENV_VARIABLES))
-
-
 NEW_RELIC_DEVELOPER_MODE = env.bool("NEW_RELIC_DEVELOPER_MODE", default=True if DEBUG else False)
 
 # Papertrail logs WARNING messages. This env variable allows modifying the behaviour