Remove isinstance() from robotparser2.py

Originally for encoding Python 2 Unicode strings [1]. Will not be used
in Python 3 because the variables are strings, if they were bytes
exceptions would be raised.

[1] c97f68f7 ("accept unicode in robots.txt can_fetch", 2004-11-09)

linkcheck/robotparser2.py

@@ -276,10 +276,6 @@ class RobotFileParser:
             useragent,
             url,
         )
-        if not isinstance(useragent, str):
-            useragent = useragent.encode("ascii", "ignore")
-        if not isinstance(url, str):
-            url = url.encode("ascii", "ignore")
         if self.disallow_all:
             log.debug(LOG_CHECK, " ... disallow all.")
             return False