Possible assertion triggered in SliceFromReadCommand

In SliceFromReadCommand.maybeGenerateRetryCommand, the following assertion

assert maxLiveColumns <= count;

may actually fail. Namely, it asserts that no node has returned more columns that what was asked for, which in general is true, but can not be if an expiring column is counted as dead by the replica (but still send as a tombstone) but, due to clock difference, is actually counted live by the coordinator.

I note that this is similar to CASSANDRA-5149 in that fixing CASSANDRA-5149 would fix this too, but in the meantime, this edge case is harmless so there is probably not much point in keeping the assertion.