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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R: @bvolpato for label java.
R: @chamikaramj for label io.
R: @igorbernstein2 for label bigtable.

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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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R: @damondouglas for label io.
R: @igorbernstein2 for label bigtable.

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R: @Abacn for label java.
R: @chamikaramj for label io.
R: @igorbernstein2 for label bigtable.

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

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R: @igorbernstein2 for label bigtable.

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[damondouglas]

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

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

[damondouglas]

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

[github-actions]

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

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Assigning new set of reviewers because Pr has gone too long without review. If you would like to opt out of this review, comment assign to next reviewer:

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R: @chamikaramj for label io.
R: @igorbernstein2 for label bigtable.

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[Abacn]

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

[github-actions]

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