crypto: remove check for condition that is always true

The value of `key` will always be `!= null` because earlier in the
function, `ERR_CRYPTO_SIGN_KEY_REQUIRED` is thrown if `key` is falsy.
Remove condition that subsequently checks that `key != null`.

PR-URL: #38072
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/crypto/sig.js

@@ -177,7 +177,7 @@ function signOneShot(algorithm, data, key, callback) {
   let keyData;
   if (isKeyObject(key) || isCryptoKey(key)) {
     ({ data: keyData } = preparePrivateKey(key));
-  } else if (key != null && (isKeyObject(key.key) || isCryptoKey(key.key))) {
+  } else if (isKeyObject(key.key) || isCryptoKey(key.key)) {
     ({ data: keyData } = preparePrivateKey(key.key));
   } else {
     keyData = createPrivateKey(key)[kHandle];