Any properly drafted will will contain these words: "I hereby revoke all former Wills and testamentary dispositions made by me" (or something similar).
So, as you suggest, HSBC are storing a meaningless piece of paper. You could (as you again suggest) simply ignore the letter.
However I'd recommend getting HSBC to send you the old document, so that you can destroy it. There are two reasons for this:
Firstly, it's unwise to complicate things by having several 'wills' (even though only one is actually valid) lying around. An executor might get hold of a revoked will and (not knowing that a later will exists) seek probate under the terms of that will. If the later will then surfaced, things could get very tricky indeed.
Secondly, it's quite likely that HSBC are charging you for storage!
Chris