Temporarily remove the trace support for RegionScanner.next

Please see the test result in HBASE-25658, tracing RegionScanner.next could lead to a 6% performance downgrading even if we do not enable tracing.

I think the problem here is that, a single scan rpc call could lead to a bunch of RegionScanner.next. So if trace is enabled, there will be much more performance impact as we need to store a bunch of spans(maybe this could be limited by some parameters?).

Anyway, let's remove it for now to land the basic trace framework first. Can improve later.