Remove FF ci_include_multiple_files_from_project

It was enabled by default

config/feature_flags/development/ci_include_multiple_files_from_project.yml

----
-name: ci_include_multiple_files_from_project
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45991
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/271560
-milestone: '13.6'
-type: development
-group: group::pipeline authoring
-default_enabled: true

doc/ci/yaml/README.md

@@ -445,10 +445,7 @@ You can use local (relative to target project), project, remote, or template inc
 ##### Multiple files from a project
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/26793) in GitLab 13.6.
-> - It's [deployed behind a feature flag](../../user/feature_flags.md), enabled by default.
-> - It's enabled on GitLab.com.
-> - It's recommended for production use.
-> - For GitLab self-managed instances, GitLab administrators can opt to disable it. **(CORE ONLY)**
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/271560) in GitLab 13.8.
 
 You can include multiple files from the same project:
 
@@ -461,23 +458,6 @@ include:
       - '/templates/.tests.yml'
 ```
 
-Including multiple files from the same project is under development but ready for production use. It is
-deployed behind a feature flag that is **enabled by default**.
-[GitLab administrators with access to the GitLab Rails console](../../administration/feature_flags.md)
-can opt to disable it.
-
-To enable it:
-
-```ruby
-Feature.enable(:ci_include_multiple_files_from_project)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:ci_include_multiple_files_from_project)
-```
-
 #### `include:remote`
 
 `include:remote` can be used to include a file from a different location,

lib/gitlab/ci/config/external/mapper.rb

@@ -54,7 +54,6 @@ module Gitlab
           end
 
           def expand_project_files(location)
-            return location unless ::Feature.enabled?(:ci_include_multiple_files_from_project, context.project, default_enabled: true)
             return location unless location[:project]
 
             Array.wrap(location[:file]).map do |file|

spec/lib/gitlab/ci/config/external/mapper_spec.rb

@@ -124,17 +124,6 @@ RSpec.describe Gitlab::Ci::Config::External::Mapper do
             an_instance_of(Gitlab::Ci::Config::External::File::Project),
             an_instance_of(Gitlab::Ci::Config::External::File::Project))
         end
-
-        context 'when FF ci_include_multiple_files_from_project is disabled' do
-          before do
-            stub_feature_flags(ci_include_multiple_files_from_project: false)
-          end
-
-          it 'returns a File instance' do
-            expect(subject).to contain_exactly(
-              an_instance_of(Gitlab::Ci::Config::External::File::Project))
-          end
-        end
       end
     end
 

spec/lib/gitlab/ci/config/external/processor_spec.rb

@@ -365,19 +365,6 @@ RSpec.describe Gitlab::Ci::Config::External::Processor do
         output = processor.perform
         expect(output.keys).to match_array([:image, :my_build, :my_test])
       end
-
-      context 'when FF ci_include_multiple_files_from_project is disabled' do
-        before do
-          stub_feature_flags(ci_include_multiple_files_from_project: false)
-        end
-
-        it 'raises an error' do
-          expect { processor.perform }.to raise_error(
-            described_class::IncludeError,
-            'Included file `["/templates/my-build.yml", "/templates/my-test.yml"]` needs to be a string'
-          )
-        end
-      end
     end
   end
 end