Remove unused files

app/helpers/assignments_helper.rb

app/helpers/conferences_helper.rb

app/helpers/sessions_helper.rb

app/helpers/speakers_helper.rb

app/helpers/users_helper.rb

app/javascript/controllers/hello_controller.js

app/mailers/application_mailer.rb

app/views/assignments/show.html.erb

app/views/filedrop_files/download.html.erb

app/views/layouts/mailer.html.erb

app/views/layouts/mailer.text.erb

app/views/sessions/index.html.erb

app/views/users/login.html.erb

app/views/users/logout.html.erb

app/views/users/profile.html.erb

app/views/users/update.html.erb

test/mailers/.keep