Update GenerateKeyEd25519 to only generate a private key

[qmuntal]

While integrating Ed25519 into the standard library I noticed that it is not necessary for GenerateKeyEd25519 to return the public key, as it can already be derived from the private key.

Doing so can help reduces allocations and cgo calls in ed25519.GenerateKey, depending on how one decides to implement it.

goos: windows
goarch: amd64
pkg: github.com/golang-fips/openssl/v2
cpu: Intel(R) Core(TM) i7-10850H CPU @ 2.70GHz
                      │   old.txt    │               new.txt               │
                      │    sec/op    │   sec/op     vs base                │
Ed25519GenerateKey-12   47.48µ ± 18%   35.97µ ± 7%  -24.25% (p=0.000 n=10)

                      │  old.txt   │              new.txt               │
                      │    B/op    │    B/op     vs base                │
Ed25519GenerateKey-12   56.00 ± 0%   16.00 ± 0%  -71.43% (p=0.000 n=10)

                      │  old.txt   │              new.txt               │
                      │ allocs/op  │ allocs/op   vs base                │
Ed25519GenerateKey-12   4.000 ± 0%   2.000 ± 0%  -50.00% (p=0.000 n=10)

[ueno]

Looks like a significant speed-up; LGTM!