Remove invite email experiment entry

This is a leftover and should have been removed with the MR that
introduced the feature.

lib/gitlab/experimentation.rb

@@ -65,10 +65,6 @@ module Gitlab
         tracking_category: 'Growth::Expansion::Experiment::CustomizeHomepage',
         use_backwards_compatible_subject_index: true
       },
-      invite_email: {
-        tracking_category: 'Growth::Acquisition::Experiment::InviteEmail',
-        use_backwards_compatible_subject_index: true
-      },
       group_only_trials: {
         tracking_category: 'Growth::Conversion::Experiment::GroupOnlyTrials',
         use_backwards_compatible_subject_index: true

spec/lib/gitlab/experimentation_spec.rb

@@ -15,7 +15,6 @@ RSpec.describe Gitlab::Experimentation::EXPERIMENTS do
       :invite_members_empty_group_version_a,
       :contact_sales_btn_in_app,
       :customize_homepage,
-      :invite_email,
       :group_only_trials,
       :default_to_issues_board
     ]