Remove LET sink expressions

Flume Config's let statement does not have sane semantics

Let was added to the config language to save resources in the DFO retry case. This however does not have well defined behavior when there are failures that occur. For example, if we had

let foo := flakey in < foo ? { deco =>

} >

While the semantics seem straight forward in the successful case (one open on flakey at the reference point, and then good to go), there could be many opens and close calls on the foo reference in cases where there are errors.

One solution would reference count the foo. To have clear semantics, we would need likely need to tighten the close semantics so that only one close is allowed per open.

Another is to just ignore or not use the let expression (Except for perhaps in testing situations).