feat(abstract-utxo)!: default to wasm-utxo for all coins

Change defaultSdkBackend from a getter to a property with constant
value 'wasm-utxo', removing network-based logic that favored utxolib
for mainnet coins.

BREAKING CHANGE: mainnet coins now default to wasm-utxo instead of
utxolib

Co-authored-by: llm-git <llm-git@ttll.de>

Issue: BTC-2650

Author: OttoAllmendinger

modules/abstract-utxo/src/abstractUtxoCoin.ts

@@ -81,7 +81,6 @@ import {
   getMainnetCoinName,
   getNetworkFromCoinName,
   isMainnetCoin,
-  isUtxoCoinNameMainnet,
   UtxoCoinName,
   UtxoCoinNameMainnet,
 } from './names';
@@ -434,9 +433,7 @@ export abstract class AbstractUtxoCoin
     this.amountType = amountType;
   }
 
-  get defaultSdkBackend(): SdkBackend {
-    return isUtxoCoinNameMainnet(this.name) ? 'utxolib' : 'wasm-utxo';
-  }
+  defaultSdkBackend: SdkBackend = 'wasm-utxo';
 
   /**
    * @deprecated - will be removed when we drop support for utxolib