fix: Remove unneeded .get()

This removes the repeated .get(). We can use the challenge_files from
the previous get() on line 764 as it remains unchanged.

Signed-off-by: AlexNg <contact@ngjx.org>

ctfcli/core/challenge.py

@@ -771,7 +771,6 @@ def lint(self, skip_hadolint=False, flag_format="flag{") -> bool:
                 )
 
         # Check that files don't have a flag in them
-        challenge_files = challenge.get("files", [])
         for challenge_file in challenge_files:
             challenge_file_path = self.challenge_directory / challenge_file