keys: remove libsecp256k1 verification until it's actually supported

This was added a while ago for testing purposes, but was never intended to be
used. Remove it until upstream libsecp256k1 decides that verification is
stable/ready.

src/key.cpp

@@ -208,11 +208,9 @@ void CExtKey::Decode(const unsigned char code[74]) {
 }
 
 bool ECC_InitSanityCheck() {
-#if !defined(USE_SECP256K1)
     if (!CECKey::SanityCheck()) {
         return false;
     }
-#endif
     CKey key;
     key.MakeNewKey(true);
     CPubKey pubkey = key.GetPubKey();