Remove function_logger

It doesn't work properly (call methods with an additional "None"
argument).

app/helpers.py

@@ -69,24 +69,6 @@ def file_is_readable(path: str) -> bool:
     return os.access(path, os.R_OK)
 
 
-def function_logger(prefix):
-    """
-    Log function parameters and return
-
-    Source: https://stackoverflow.com/questions/25936746/
-        create-a-function-decorator-that-logs-arguments
-    """
-
-    def decorate(f):
-        def wrapper(*args, **kwargs):
-            print(prefix, f.__name__, "args", args, "kwargs", kwargs)
-            cr = f(*args, **kwargs)
-            print(prefix, f.__name__, "result", cr)
-            return cr
-        return wrapper
-    return decorate
-
-
 def get_audio_segment(path: str) -> Optional[AudioSegment]:
     try:
         if path.endswith('.mp3'):