Clean up stale code in BigtableService

[mutianf]

The underlying issue with attempt timeout is fixed in the Bigtable client. We don't need to pass around operation and attempt timeout in the reader anymore. This PR cleans up the code that's not used anymore.

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[Abacn]

Are these "stale code" not used in Beam, i.e., there is currently no way to make this code in effect; or it's not needed? If it still can make some effect on Beam I would suggests defer the clean up until we see the issue no longer exist for a time period

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Reminder, please take a look at this pr: @kennknowles @damondouglas @igorbernstein2

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Reminder, please take a look at this pr: @Abacn @chamikaramj @igorbernstein2

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[damondouglas]

@mutianf I'll have a look at it. Apologies for your wait.

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[damondouglas]

@mutianf LGTM, would you like me to merge this?

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[Abacn]

This one has taken long. Going to merge it in 24 hrs if no other concern

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