Remove custom Danger check for frozen_string_literal comments

This is redundant with Rubocop's Style/FrozenStringLiteralComment cop,
which we didn't have enabled when this was introduced.

To preserve the same behaviour we also:

- Switch to `EnforcedStyle: always_true`, to forbid `false` values.
- Replace the wildcard list with an explicit list of all violations,
  so we still catch all newly added files.

.rubocop.yml

@@ -78,21 +78,7 @@ Style/AccessModifierDeclarations:
 # Frozen String Literal
 Style/FrozenStringLiteralComment:
   Enabled: true
-  Exclude:
-    - 'config.ru'
-    - 'Dangerfile'
-    - 'Gemfile'
-    - 'Rakefile'
-    - 'app/views/**/*'
-    - 'config/**/*'
-    - 'danger/**/*'
-    - 'db/**/*'
-    - 'ee/db/**/*'
-    - 'ee/lib/tasks/**/*'
-    - 'lib/tasks/**/*'
-    - 'qa/**/*'
-    - 'rubocop/**/*'
-    - 'scripts/**/*'
+  EnforcedStyle: always_true
 
 RSpec/FilePath:
   Exclude:

danger/frozen_string/Dangerfile

-# frozen_string_literal: true
-
-FILE_EXTENSION = ".rb"
-FROZEN_STRING_MAGIC_COMMENT = "# frozen_string_literal: true"
-SHEBANG_COMMENT = "#!"
-
-def get_files_with_no_magic_comment(files)
-  files.select do |path|
-    path.end_with?(FILE_EXTENSION) &&
-      !file_has_frozen_string_magic_comment?(path)
-  end
-end
-
-def file_has_frozen_string_magic_comment?(path)
-  File.open(path) do |file|
-    first_line = file.gets
-
-    line_has_frozen_string_magic_comment?(first_line) ||
-      (line_has_shebang?(first_line) &&
-        line_has_frozen_string_magic_comment?(file.gets))
-  end
-end
-
-def line_has_frozen_string_magic_comment?(line)
-  line&.start_with?(FROZEN_STRING_MAGIC_COMMENT)
-end
-
-def line_has_shebang?(line)
-  line&.start_with?(SHEBANG_COMMENT)
-end
-
-files_to_fix = get_files_with_no_magic_comment(git.added_files)
-
-if files_to_fix.any?
-  warn 'This merge request adds files that do not enforce frozen string literal. ' \
-    'See https://gitlab.com/gitlab-org/gitlab-foss/issues/47424 for more information.'
-
-  if GitlabDanger.new(helper.gitlab_helper).ci?
-    markdown(<<~MARKDOWN)
-      ## Enable Frozen String Literal
-
-      The following files should have `#{FROZEN_STRING_MAGIC_COMMENT}` on the first line:
-
-      * #{files_to_fix.map { |path| "`#{path}`" }.join("\n* ")}
-    MARKDOWN
-  end
-end

lib/gitlab_danger.rb

@@ -4,7 +4,6 @@ class GitlabDanger
   LOCAL_RULES ||= %w[
     changes_size
     documentation
-    frozen_string
     duplicate_yarn_dependencies
     prettier
     eslint