net: merge V2Transport constructors, move key gen

This removes the ability for BIP324Cipher to generate its own key, moving that
responsibility to the caller (mostly, V2Transport). This allows us to write
the random-key V2Transport constructor by delegating to the explicit-key one.

src/bip324.cpp

@@ -22,13 +22,6 @@
 #include <iterator>
 #include <string>
 
-BIP324Cipher::BIP324Cipher() noexcept
-{
-    m_key.MakeNewKey(true);
-    uint256 entropy = GetRandHash();
-    m_our_pubkey = m_key.EllSwiftCreate(MakeByteSpan(entropy));
-}
-
 BIP324Cipher::BIP324Cipher(const CKey& key, Span<const std::byte> ent32) noexcept :
     m_key(key)
 {