Remove ci_variable_expansion_in_rules_changes feature flag

Remove feature flag file
Remove feature flag guards
Update documentation

config/feature_flags/development/ci_variable_expansion_in_rules_changes.yml

----
-name: ci_variable_expansion_in_rules_changes
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45037
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/267192
-type: development
-group: group::pipeline authoring
-default_enabled: true

doc/ci/yaml/README.md

@@ -1342,14 +1342,7 @@ if there is no `if:` statement that limits the job to branch or merge request pi
 ##### Variables in `rules:changes`
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/34272) in GitLab 13.6.
-> - It was [deployed behind a feature flag](../../user/feature_flags.md), disabled by default.
-> - [Became enabled by default](https://gitlab.com/gitlab-org/gitlab/-/issues/267192) in GitLab 13.6.
-> - It's enabled on GitLab.com.
-> - It's recommended for production use.
-> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-variables-support-in-ruleschanges). **(CORE ONLY)**
-
-CAUTION: **Warning:**
-This feature might not be available to you. Check the **version history** note above for details.
+> - [Feature flag removed](https://gitlab.com/gitlab-org/gitlab/-/issues/267192) in GitLab 13.7.
 
 Environment variables can be used in `rules:changes` expressions to determine when
 to add jobs to a pipeline:
@@ -1368,25 +1361,6 @@ The `$` character can be used for both variables and paths. For example, if the
 `$DOCKERFILES_DIR` variable exists, its value is used. If it does not exist, the
 `$` is interpreted as being part of a path.
 
-###### Enable or disable variables support in `rules:changes` **(CORE ONLY)**
-
-Variables support in `rules:changes` is under development, but is 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_variable_expansion_in_rules_changes)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:ci_variable_expansion_in_rules_changes)
-```
-
 #### `rules:exists`
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/24021) in GitLab 12.4.

lib/gitlab/ci/build/rules/rule/clause/changes.rb

@@ -11,7 +11,7 @@ module Gitlab
         def satisfied_by?(pipeline, context)
           return true if pipeline.modified_paths.nil?
 
-          expanded_globs = expand_globs(pipeline, context)
+          expanded_globs = expand_globs(context)
           pipeline.modified_paths.any? do |path|
             expanded_globs.any? do |glob|
               File.fnmatch?(glob, path, File::FNM_PATHNAME | File::FNM_DOTMATCH | File::FNM_EXTGLOB)
@@ -19,8 +19,7 @@ module Gitlab
           end
         end
 
-        def expand_globs(pipeline, context)
-          return @globs unless ::Feature.enabled?(:ci_variable_expansion_in_rules_changes, pipeline.project, default_enabled: true)
+        def expand_globs(context)
           return @globs unless context
 
           @globs.map do |glob|