Rollup merge of #51186 - pnkfelix:remove-unneccessary-nll-stderr-files, r=oli-obk

Remove two redundant .nll.stderr files

It turns out that the diagnostics generated from NLL for these cases are now exactly the same as that produced by AST borrowck, and thus we can just fallback on those `.stderr` files that already exist for AST-borrowck.

Bravo!

(it is a good idea to remove these files, because it slightly reduces the amount of time humans will spend reviewing the .nll.stderr fileset...)

((it *might* be worthwhile trying to change the `compiletest` code to even issue a warning when two such files have equivalent contents... but I am not going so far as to try to implement that right now...))

Author: kennytm