Remove testBlockValidity() from mining interface

It's very low level and not used by the proposed Template Provider.

This method was introduced in d8a3496b5a
and a74b0f93ef.

test/functional/rpc_generate.py

@@ -87,7 +87,7 @@ class RPCGenerateTest(BitcoinTestFramework):
         txid1 = miniwallet.send_self_transfer(from_node=node)['txid']
         utxo1 = miniwallet.get_utxo(txid=txid1)
         rawtx2 = miniwallet.create_self_transfer(utxo_to_spend=utxo1)['hex']
-        assert_raises_rpc_error(-25, 'testBlockValidity failed: bad-txns-inputs-missingorspent', self.generateblock, node, address, [rawtx2, txid1])
+        assert_raises_rpc_error(-25, 'TestBlockValidity failed: bad-txns-inputs-missingorspent', self.generateblock, node, address, [rawtx2, txid1])
 
         self.log.info('Fail to generate block with txid not in mempool')
         missing_txid = '0000000000000000000000000000000000000000000000000000000000000000'