Get rid of unused builtin_str

Author: byroot

pysrt/compat.py

@@ -13,12 +13,10 @@
 from io import open as io_open
 
 if is_py2:
-    builtin_str = str
     basestring = basestring
     str = unicode
     open = io_open
 elif is_py3:
-    builtin_str = str
     basestring = (str, bytes)
     str = str
     open = open