Remove ci_hidden_variables ff

...pts/ci/ci_variable_list/components/ci_variable_shared.vue

app/controllers/groups/settings/ci_cd_controller.rb

app/controllers/projects/settings/ci_cd_controller.rb

app/models/ci/variable_value.rb

app/models/concerns/ci/hidable_variable.rb

app/views/ci/variables/_attributes.html.haml

config/feature_flags/development/ci_hidden_variables.yml

doc/ci/variables/index.md

spec/models/ci/variable_value_spec.rb

spec/models/concerns/ci/hidable_variable_spec.rb

spec/requests/api/ci/variables_spec.rb

spec/requests/api/graphql/ci/group_variables_spec.rb

spec/requests/api/graphql/ci/project_variables_spec.rb

spec/requests/api/group_variables_spec.rb