Deleted gcb2slack (#3849)

We're not using this on the latest Slack channel, and this code requires maintenance that has no reward.

Author: mhutchinson

.github/dependabot.yml

@@ -39,8 +39,3 @@ updates:
       applies-to: version-updates
       patterns:
         - "*"
-
-- package-ecosystem: npm
-  directory: /scripts/gcb2slack
-  schedule:
-    interval: monthly

CHANGELOG.md

@@ -8,6 +8,7 @@
 * Add TLS support for PostgreSQL: https://github.com/google/trillian/pull/3831
   * `--postgresql_tls_ca`: users can provide a CA certificate, that is used to establish a secure communication with PostgreSQL server. 
   * `--postgresql_verify_full`: users can enable full TLS verification for PostgreSQL (sslmode=verify-full). If false, only sslmode=verify-ca is used.
+* Deleted `gcb2slack`
 
 ### Storage