Minor Salesforce runner fix

[ccakes]

Issue Summary

A Security Token isn't required in all SFDC environments - depending on configuration. See here for more information.

Steps to Reproduce

1. Add Salesforce as a data source where a token isn't required (and cannot be generated)
2. Cannot proceed without required field

Technical details:

redash/redash/query_runner/salesforce.py

Line 81 in be56035

"required": ["username", "password", "token"],

Just remove token from the required list. Seemed like it'd be faster to create an issue than submit a PR for such a small change

[ccakes]

Might be worth noting that I'm running a version with the change applied and it works 👌