Remove rspec-set gem

We've removed all instances of `set` and have moved them all over to
use `let_it_be`

Gemfile

@@ -399,7 +399,6 @@ group :test do
   gem 'fuubar', '~> 2.2.0'
   gem 'rspec-retry', '~> 0.6.1'
   gem 'rspec_profiling', '~> 0.0.5'
-  gem 'rspec-set', '~> 0.1.3'
   gem 'rspec-parameterized', require: false
 
   gem 'capybara', '~> 3.22.0'

Gemfile.lock

@@ -923,7 +923,6 @@ GEM
       rspec-support (~> 3.9)
     rspec-retry (0.6.1)
       rspec-core (> 3.3)
-    rspec-set (0.1.3)
     rspec-support (3.9.2)
     rspec_junit_formatter (0.4.1)
       rspec-core (>= 2, < 4, != 2.12.0)
@@ -1353,7 +1352,6 @@ DEPENDENCIES
   rspec-parameterized
   rspec-rails (~> 4.0.0.beta4)
   rspec-retry (~> 0.6.1)
-  rspec-set (~> 0.1.3)
   rspec_junit_formatter
   rspec_profiling (~> 0.0.5)
   rubocop (~> 0.74.0)

doc/development/testing_guide/best_practices.md

@@ -278,26 +278,6 @@ new object.
 let_it_be(:project, refind: true) { create(:project) }
 ```
 
-### `set` variables
-
-NOTE: **Note:**
-We are incrementally removing `set` in favour of `let_it_be`. See the
-[removal issue](https://gitlab.com/gitlab-org/gitlab/issues/27922).
-
-In some cases there is no need to recreate the same object for tests again for
-each example. For example, a project is needed to test issues on the same
-project, one project will do for the entire file. This can be achieved by using
-`set` in the same way you would use `let`.
-
-`rspec-set` only works on ActiveRecord objects, and before new examples it
-reloads or recreates the model, _only_ if needed. That is, when you changed
-properties or destroyed the object.
-
-Note that you can't reference a model defined in a `let` block in a `set` block.
-
-Also, `set` is not supported in `:js` specs since those don't use transactions
-to clean up database state after each example.
-
 ### Time-sensitive tests
 
 [Timecop](https://github.com/travisjeffery/timecop) is available in our

spec/lib/gitlab/alerting/alert_spec.rb

@@ -211,9 +211,7 @@ describe Gitlab::Alerting::Alert do
     it { is_expected.to be_valid }
 
     context 'without project' do
-      # Redefine to prevent:
-      # project is a NilClass - rspec-set works with ActiveRecord models only
-      let(:alert) { build(:alerting_alert, project: nil, payload: payload) }
+      let(:project) { nil }
 
       it { is_expected.not_to be_valid }
     end