Remove split dashboard buttons due to low usage

...ets/javascripts/pages/admin/application_settings/index.js

app/assets/javascripts/pages/dashboard/issues/index.js

...ssets/javascripts/pages/dashboard/merge_requests/index.js

...ets/javascripts/pages/dashboard/milestones/index/index.js

app/assets/javascripts/pages/groups/issues/index.js

app/assets/javascripts/pages/groups/merge_requests/index.js

app/views/dashboard/issues.html.haml

app/views/dashboard/merge_requests.html.haml

app/views/dashboard/milestones/index.html.haml

app/views/groups/issues.html.haml

app/views/groups/merge_requests.html.haml

spec/features/dashboard/issues_spec.rb

spec/features/dashboard/merge_requests_spec.rb

spec/features/dashboard/milestones_spec.rb

spec/features/groups/empty_states_spec.rb

spec/features/groups/issues_spec.rb

spec/features/groups/merge_requests_spec.rb