Remove `random_markdown_tip` helper

Only usage got removed in !3442

app/helpers/gitlab_markdown_helper.rb

@@ -116,29 +116,6 @@ module GitlabMarkdownHelper
     end
   end
 
-  MARKDOWN_TIPS = [
-    "End a line with two or more spaces for a line-break, or soft-return",
-    "Inline code can be denoted by `surrounding it with backticks`",
-    "Blocks of code can be denoted by three backticks ``` or four leading spaces",
-    "Emoji can be added by :emoji_name:, for example :thumbsup:",
-    "Notify other participants using @user_name",
-    "Notify a specific group using @group_name",
-    "Notify the entire team using @all",
-    "Reference an issue using a hash, for example issue #123",
-    "Reference a merge request using an exclamation point, for example MR !123",
-    "Italicize words or phrases using *asterisks* or _underscores_",
-    "Bold words or phrases using **double asterisks** or __double underscores__",
-    "Strikethrough words or phrases using ~~two tildes~~",
-    "Make a bulleted list using + pluses, - minuses, or * asterisks",
-    "Denote blockquotes using > at the beginning of a line",
-    "Make a horizontal line using three or more hyphens ---, asterisks ***, or underscores ___"
-  ].freeze
-
-  # Returns a random markdown tip for use as a textarea placeholder
-  def random_markdown_tip
-    MARKDOWN_TIPS.sample
-  end
-
   private
 
   # Return +text+, truncated to +max_chars+ characters, excluding any HTML

spec/helpers/gitlab_markdown_helper_spec.rb

@@ -150,13 +150,6 @@ describe GitlabMarkdownHelper do
     end
   end
 
-  describe 'random_markdown_tip' do
-    it 'returns a random Markdown tip' do
-      stub_const("#{described_class}::MARKDOWN_TIPS", ['Random tip'])
-      expect(random_markdown_tip).to eq 'Random tip'
-    end
-  end
-
   describe '#first_line_in_markdown' do
     let(:text) { "@#{user.username}, can you look at this?\nHello world\n"}