Remove sidebar dead code (#23984)

app/views/application/_sidebar.html.haml

config/locales/an.yml

config/locales/ar.yml

config/locales/ast.yml

config/locales/be.yml

config/locales/bg.yml

config/locales/ca.yml

config/locales/ckb.yml

config/locales/co.yml

config/locales/cs.yml

config/locales/cy.yml

config/locales/da.yml

config/locales/de.yml

config/locales/el.yml

config/locales/en.yml

config/locales/eo.yml

config/locales/es-AR.yml

config/locales/es-MX.yml

config/locales/es.yml

config/locales/et.yml