btrfs-progs: docs: remove unused variable in sphinx config script

CodeQL reports and unused variable 'env', it's not used in the manref
formatter, probably a copy&paste leftover.

[ci skip]

Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

Documentation/conf.py

@@ -159,7 +159,6 @@ def run(self):
 # - format is strict
 # - html link target is not validated
 def role_manref(name, rawtext, text, lineno, inliner, options={}, content=[]):
-    env = inliner.document.settings.env
     name, number = text.split('(', 1)
     number = number.split(')')[0]