Bug 1434430 - [flake8] Drop support for the FLAKE8 environment variable r=rwood

The flake8 linter requires a specific version of flake8, so that running it
locally vs in CI vs in mozreview will all produce the same results.

Allowing consumers to specify a custom flake8 binary via the FLAKE8 environment
variable, subverts that goal and can result in unexplained errors showing up in
some configurations but not others.

MozReview-Commit-ID: 1s0nC8ZO6Qi

tools/lint/python/flake8.py

@@ -95,10 +95,6 @@ def get_flake8_binary():
     Returns the path of the first flake8 binary available
     if not found returns None
     """
-    binary = os.environ.get('FLAKE8')
-    if binary:
-        return binary
-
     try:
         return which.which('flake8')
     except which.WhichError: