Remove more Python 2 code

I found another more place that still had a workaround for Python 2.
This patch removes it.

gdb/python/lib/gdb/FrameDecorator.py

@@ -15,15 +15,6 @@
 
 import gdb
 
-# This small code snippet deals with problem of strings in Python 2.x
-# and Python 3.x.  Python 2.x has str and unicode classes which are
-# sub-classes of basestring.  In Python 3.x all strings are encoded
-# and basestring has been removed.
-try:
-    basestring
-except NameError:
-    basestring = str
-
 
 class FrameDecorator(object):
     """Basic implementation of a Frame Decorator"""
@@ -252,7 +243,7 @@ class FrameVars(object):
         # SYM may be a string instead of a symbol in the case of
         # synthetic local arguments or locals.  If that is the case,
         # always fetch.
-        if isinstance(sym, basestring):
+        if isinstance(sym, str):
             return True
 
         sym_type = sym.addr_class