cmd/securitybot: delete securitybot

The security TryBot infrastructure is no more.

Change-Id: I63ce14fdaebee1f0bbe5b537692a23c8be749798
Reviewed-on: https://go-review.googlesource.com/c/build/+/584415
Auto-Submit: Roland Shoemaker <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: rolandshoemaker