crypto/internal/fips/check: remove unreachable return

crypto/internal/fips/check: remove unreachable return

return statement is immediately after a panic. This is unreachable
code and vet complains.

Change-Id: I1e483a552b0d1d543414c1e173c1140aa32b5b26
Reviewed-on: https://go-review.googlesource.com/c/go/+/628555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>