Remove unnecessary check for unicode instance (we are py3+ now)

Author: kinow

pandas/_libs/writers.pyx

@@ -137,8 +137,6 @@ cpdef inline Py_ssize_t max_len_string(object val):
         l = PyString_GET_SIZE(val)
     elif isinstance(val, bytes):
         l = PyBytes_GET_SIZE(val)
-    elif isinstance(val, unicode):
-        l = PyUnicode_GET_SIZE(val)
 
     return l