Don't check if bot sessions exist in backend

Description

Currently, when permissive botsession mode is enabled, we check to make sure that the bot's leases exist in the scheduler as a sanity check to ensure that attackers can't connect to BuildGrid with arbitrary bots. However, this does not function well, as bots without leases are not an uncommon occurrence, and BuildGrid will record an error for each for these bots during a swingover. Furthermore, verifying that bots are who they say they are is best handled in an authentication layer, not in the bot service's business logic.

This MR removes this check from the bots service.