Removes enforcement of "as_user" field. Fixes #1943

[GaryTowers]

Fixes issue #1943

By removing this IF statement:

New generation bots will be able to make use of icon_url, icon_emoji and username fields on the reply object without having Botkit forcing a deprecated field as_user set to false.

For classic bots, developers would still be able to add the as_user field manually as stated by the Slack documentation

(I'm not sure if I should make a PR to master or other branch, please advice)

[benbrown]

This would break existing bots using this feature.

Please add a check for the new oauth_version configuration option - only exclude field if it is set to v2

[GaryTowers]

Updated to fix issue #1943 based on oauth_version set to v2

The v2 check before removing the as_user field makes this backwards compatible with v1

Thanks for the observation @benbrown