Generate new mnemonics with wasm-crypto again

mnemonicGenerate was forced onto the JavaScript path because
wasm-crypto 7.5.18 had no bip39Generate. 7.5.20 has it: 1000 phrases of
12 to 24 words generated through wasm were all valid and distinct, and
wasm and JavaScript derive the same mini secret from each. The call is
back to the upstream form, which uses wasm when it is ready.

packages/extension-base/src/background/handlers/Extension.ts

@@ -325,8 +325,7 @@ export default class Extension {
   }
 
   private seedCreate ({ length = SEED_DEFAULT_LENGTH, seed: _seed, type }: RequestSeedCreate): ResponseSeedCreate {
-    // Use onlyJs=true because wasm-crypto doesn't implement bip39Generate
-    const seed = _seed || mnemonicGenerate(length, undefined, true);
+    const seed = _seed || mnemonicGenerate(length);
 
     return {
       address: keyring.createFromUri(getSuri(seed, type), {}, type).address,