Removes MAX_TX_TO_IMPORT from ChainSync

[0x7CFE]

This PR removes MAX_TX_TO_IMPORT constant and its uses in sync/src/chain.rs.

As @tomusdrw explained:

…back in the past we were parsing the transactions and actually importing them to the queue which took significant amount of time. Currently we have a queue of unverifiedTransactions that are later parsed and imported to the queue in batches (the method is now called queue_transactions). So the limiting doesn't currently make sense at all.

[tomusdrw]

Those transactions get queued in the client anyway and then processed in batches.
Currently we were discarding some transactions (if there was more than 512 in single packet)