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>

tests/server/test_mempool.py

@@ -12,13 +12,13 @@ import pytest
 from aiorpcx import Event, sleep, ignore_after
 
 from electrumx.server.mempool import MemPool, MemPoolAPI
-from electrumx.lib.coins import BitcoinCash
+from electrumx.lib.coins import Palladium
 from electrumx.lib.hash import HASHX_LEN, hex_str_to_hash, hash_to_hex_str
 from electrumx.lib.tx import Tx, TxInput, TxOutput
 from electrumx.lib.util import OldTaskGroup
 
 
-coin = BitcoinCash
+coin = Palladium
 tx_hash_fn = coin.DESERIALIZER.TX_HASH_FN
 # Change seed daily
 seed(datetime.date.today().toordinal())