Remove ci_parallel_matrix_enabled feature flag

Enables ci_parallel_matrix_enabled feature flag by removing it

doc/ci/yaml/README.md

@@ -3473,10 +3473,6 @@ job split into three separate jobs.
 #### Parallel `matrix` jobs
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/15356) in GitLab 13.3.
-> - It's deployed behind a feature flag, disabled by default.
-> - It's enabled on GitLab.com.
-> - It can't be enabled or disabled per-project.
-> - It's recommended for production use.
 
 `matrix:` allows you to configure different variables for jobs that are running in parallel.
 There can be from 2 to 50 jobs.

lib/gitlab/ci/config/entry/product/parallel.rb

@@ -10,7 +10,7 @@ module Gitlab
         module Product
           class Parallel < ::Gitlab::Config::Entry::Simplifiable
             strategy :ParallelBuilds, if: -> (config) { config.is_a?(Numeric) }
-            strategy :MatrixBuilds, if: -> (config) { ::Gitlab::Ci::Features.parallel_matrix_enabled? && config.is_a?(Hash) }
+            strategy :MatrixBuilds, if: -> (config) { config.is_a?(Hash) }
 
             PARALLEL_LIMIT = 50
 

lib/gitlab/ci/config/normalizer/factory.rb

@@ -29,11 +29,7 @@ module Gitlab
           end
 
           def strategies
-            if ::Gitlab::Ci::Features.parallel_matrix_enabled?
             [NumberStrategy, MatrixStrategy]
-            else
-              [NumberStrategy]
-            end
           end
         end
       end

lib/gitlab/ci/features.rb

@@ -62,10 +62,6 @@ module Gitlab
         ::Feature.enabled?(:destroy_only_unlocked_expired_artifacts, default_enabled: false)
       end
 
-      def self.parallel_matrix_enabled?
-        ::Feature.enabled?(:ci_parallel_matrix_enabled)
-      end
-
       def self.bulk_insert_on_create?(project)
         ::Feature.enabled?(:ci_bulk_insert_on_create, project, default_enabled: true)
       end