Remove Dangerfile for prettier

This Dangerfile dates back to when we gradually adopted prettier. Now
our code base is fully prettier compliant and we have a separate
static-analysis job for prettier _and_ we run it part of lefthook. So
this Danger job just adds noise to the already long danger output and
potentially takes a long time.

Let's remove it, it outlived it's purpose.

danger/prettier/Dangerfile

-# frozen_string_literal: true
-
-def get_prettier_files(files)
-  files.select do |file|
-    file.end_with?('.js', '.scss', '.vue')
-  end
-end
-
-prettier_candidates = get_prettier_files(helper.all_changed_files)
-
-return if prettier_candidates.empty?
-
-unpretty = `node_modules/prettier/bin-prettier.js --list-different #{prettier_candidates.join(" ")}`
-             .split(/$/)
-             .map(&:strip)
-             .reject(&:empty?)
-
-return if unpretty.empty?
-
-warn 'This merge request changed frontend files without pretty printing them.'
-
-if helper.ci?
-  markdown(<<~MARKDOWN)
-    ## Pretty print Frontend files
-
-    The following files should have been pretty printed with `prettier`:
-
-    * #{unpretty.map { |path| "`#{path}`" }.join("\n* ")}
-
-    Please run
-
-    ```
-    node_modules/.bin/prettier --write \\
-    #{unpretty.map { |path| "  '#{path}'" }.join(" \\\n")}
-    ```
-
-    Also consider auto-formatting [on-save].
-
-    [on-save]: https://docs.gitlab.com/ee/development/fe_guide/tooling.html#formatting-with-prettier
-  MARKDOWN
-end