Remove all coins except Palladium

coins.py keeps only Coin and the Palladium classes. The Bitcoin base class is
folded into Palladium. COIN now defaults to Palladium.

Tests and fixtures of the other coins are removed. test_blocks.py and
test_transactions.py are simplified: one test each, fed by the palladium
fixtures.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/electrumx/server/env.py

@@ -52,7 +52,7 @@ class Env(EnvBase):
             assert issubclass(coin, Coin)
             self.coin = coin
         else:
-            coin_name = self.default('COIN', 'Bitcoin').strip()
+            coin_name = self.default('COIN', 'Palladium').strip()
             network = self.default('NET', 'mainnet').strip()
             self.coin = Coin.lookup_coin_class(coin_name, network)