subtle.ConstantTimeCompare now tests the length of the inputs (although
it didn't when this code was written) so this test in crypto/hmac is now
superfluous.

Fixes #16336.

Change-Id: Ic02d8537e776fa1dd5694d3af07a28c4d840d14b
Reviewed-on: https://go-review.googlesource.com/27239Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>