>> Fortunately, Emacs already has such a command named ‘delete-pair’:
>> it deletes the parens around the let function as in your first
>> example as well as the quotes in your second example.  It makes
>> sense to bind this command to e.g. ‘C-x M-(’ to be the inverse of
>> ‘M-(’ (insert-pair)
>
> Yay! I didn't know about that command (and I really did search before
> reporting this bug, no idea how I missed it!). It doesn't work backward
> though as far as I can tell:
>
> (let (message "%s" "foobar")|)

With a simple twist in the patch attached, it will be able to work backward
with M-- M-x delete-pair, i.e. with a negative prefix argument.