Replace let with let_it_be

Improve some specs by using `let_it_be`

lib/gitlab/slash_commands/presenters/issue_comment.rb

@@ -27,7 +27,6 @@ module Gitlab
                 mrkdwn_in: [
                   :title,
                   :pretext,
-                  :text,
                   :fields
                 ]
               }

spec/lib/gitlab/slash_commands/issue_comment_spec.rb

@@ -4,8 +4,8 @@ require 'spec_helper'
 
 describe Gitlab::SlashCommands::IssueComment do
   describe '#execute' do
-    let(:issue) { create(:issue, project: project) }
-    let(:project) { create(:project) }
+    let_it_be(:project) { create(:project) }
+    let_it_be(:issue) { create(:issue, project: project) }
     let(:user) { issue.author }
     let(:chat_name) { double(:chat_name, user: user) }
     let(:regex_match) { described_class.match("issue comment #{issue.iid}\nComment body") }

spec/lib/gitlab/slash_commands/presenters/issue_comment_spec.rb

@@ -3,9 +3,9 @@
 require 'spec_helper'
 
 describe Gitlab::SlashCommands::Presenters::IssueComment do
-  let(:project) { create(:project) }
-  let(:issue) { create(:issue, project: project) }
-  let(:note) { create(:note, project: project, noteable: issue) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:issue) { create(:issue, project: project) }
+  let_it_be(:note) { create(:note, project: project, noteable: issue) }
   let(:author) { note.author }
 
   describe '#present' do