Summary:
The `non_transactional_buffered_write` field in the `PgPerformOptionsPB` was introduced in 807d224b8825 / D42271 to force read time selection on docDB in case of writes without distributed transaction. This field is not necessary to achieve required behavior. Because `PgPerformOptionsPB` allows to reset read time on a session by setting the `read_time` field to an empty value.

This diff removes the `non_transactional_buffered_write` field (almost reverts 807d224b8825 / D42271) and uses read time reset mechanism instead.

Upgrade/Rollback safety:
The field 'non_transactional_buffered_write' is removed from the `PgPerformOptionsPB` in `pg_client.proto`. This protobuf is only used between pggate and local t-server. Any modifications of this protobuf is upgrade/rollback safe.
Jira: DB-15550

Original commit: 93bbed7f112a4153ac46ec173cbe2805c3459152 / D43281

Test Plan: Jenkins

Reviewers: patnaik.balivada, pjain, rthallam, huapeng.yuan

Reviewed By: patnaik.balivada

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D45914