Remove Slack notifications for CI failures

We were storing the Slack secrets on a CircleCI context [1]. Although we
were also passing them to forks [2], it resulted on unauthorized builds
for external contributions.

We could work around the issue in two ways:

- Having the secrets outside of any context, but that would compromise
 the security of the associated Slack channel for:
  - Send messages as @circleci notifications
  - Send messages to channels @circleci notifications isn't a member of
  - Upload, edit, and delete files as CircleCI notifications
- Using CircleCI logic statements [3] to conditionally run jobs when
`CIRCLECI_USERNAME` or `CIRCLE_PR_USERNAME` env vars [4] are in a list
of allowed users. However, that would be something difficult to
maintain, and there's no other way to check the user's role.

Given that we don't find those trade-offs to be acceptable, we remove
the integration for now.

[1] - https://circleci.com/docs/contexts/
[2] - https://circleci.com/docs/oss/#pass-secrets-to-builds-from-forked-pull-requests
[3] - https://circleci.com/docs/configuration-reference/#logic-statements
[4] - https://circleci.com/docs/variables/

Author: waiting-for-dev

.circleci/config.yml

@@ -6,7 +6,6 @@ orbs:
   # to change this configuration every time a Solidus version is released
   # or goes EOL.
   solidusio_extensions: solidusio/extensions@volatile
-  slack: circleci/slack@4.9.3
 
 commands:
   setup:
@@ -20,44 +19,33 @@ commands:
             gem install bundler -v '>=2.3.21' --conservative
             bundle --version
 
-  notify:
-    steps:
-      - slack/notify:
-          event: fail
-          template: basic_fail_1
-          branch_pattern: master
-
 jobs:
   solidus-master:
     executor:
       name: solidusio_extensions/sqlite
       ruby_version: '3.1'
-    steps: ['setup', 'solidusio_extensions/run-tests-solidus-master', 'notify']
+    steps: ['setup', 'solidusio_extensions/run-tests-solidus-master']
   solidus-current:
     executor:
       name: solidusio_extensions/sqlite
       ruby_version: '3.0'
-    steps: ['setup', 'solidusio_extensions/run-tests-solidus-current', 'notify']
+    steps: ['setup', 'solidusio_extensions/run-tests-solidus-current']
   solidus-older:
     executor:
       name: solidusio_extensions/sqlite
       ruby_version: '2.7'
-    steps: ['setup', 'solidusio_extensions/run-tests-solidus-older', 'notify']
+    steps: ['setup', 'solidusio_extensions/run-tests-solidus-older']
   lint-code:
     executor: solidusio_extensions/sqlite
-    steps: ['setup', 'solidusio_extensions/lint-code', 'notify']
+    steps: ['setup', 'solidusio_extensions/lint-code']
 
 workflows:
   "Run specs on supported Solidus versions":
     jobs:
-      - solidus-master:
-          context: slack-secrets
-      - solidus-current:
-          context: slack-secrets
-      - solidus-older:
-          context: slack-secrets
-      - lint-code:
-          context: slack-secrets
+      - solidus-master
+      - solidus-current
+      - solidus-older
+      - lint-code
 
   "Weekly run specs against master":
     triggers:
@@ -68,9 +56,6 @@ workflows:
               only:
                 - master
     jobs:
-      - solidus-master:
-          context: slack-secrets
-      - solidus-current:
-          context: slack-secrets
-      - solidus-older:
-          context: slack-secrets
+      - solidus-master
+      - solidus-current
+      - solidus-older