Remove the old 'is not' logic to detect if not instances and use them to do branch predictions. This is not necessary anymore, since we now support that in a more general way (flow_analysis).

test/completion/basic.py

@@ -356,16 +356,6 @@ foo = \
 #? int()
 foo
 
-# -----------------
-# if `is not` checks
-# -----------------
-
-foo = ['a']
-if foo is not None:
-    foo = ''.join(foo)
-#? str()
-foo
-
 # -----------------
 # module attributes
 # -----------------