chore: note Context.in_assertion is dead

the Context has an `in_assertion` flag which, when set, indicates that
the context should be constant according to is_constant() definition.
however, this flag is never set during code generation, specifically, it
is possible to have non-constant expression in an assert statement. For
example, the following contract compiles:

```
x: uint256

@internal
def bar() -> uint256:
    self.x = 1
    return self.x

@external
def foo():
    assert self.bar() == 1
```

CS-VYPER_MAY_2023-005

vyper/codegen/context.py

@@ -69,6 +69,7 @@ def __init__(
         self.constancy = constancy
 
         # Whether body is currently in an assert statement
+        # XXX: dead, never set to True
         self.in_assertion = False
 
         # Whether we are currently parsing a range expression