multishard_combining_reader: drop tracking of pending next_partition calls
multishard_combining_reader: drop tracking of pending next_partition calls

Shard readers used to track pending `next_partition()` calls that they
couldn't execute, because their underlying reader wasn't created yet.
These pending calls were then executed after the reader was created.
However the only situation where a shard reader can receive a
`next_partition()` call, before its underlying reader wasn't created is
when `next_partition()` is called on the multishard reader before a
single fragment is read. In this case we know we are at a partition
boundary and thus this call has no effect, therefore it is safe to
ignore it.

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