Remove legacy slider (#19255)

app/slider/README.md

app/slider/client/index.js

app/slider/client/rocketchat-slider.html

app/slider/client/rocketchat-slider.js

app/slider/index.js

app/theme/client/imports/components/slider.css

app/theme/client/imports/general/theme_old.css

app/theme/client/imports/general/variables.css

app/theme/client/main.css

client/importPackages.js

packages/rocketchat-i18n/i18n/af.i18n.json

packages/rocketchat-i18n/i18n/ar.i18n.json

packages/rocketchat-i18n/i18n/az.i18n.json

packages/rocketchat-i18n/i18n/be-BY.i18n.json

packages/rocketchat-i18n/i18n/bg.i18n.json

packages/rocketchat-i18n/i18n/bs.i18n.json

packages/rocketchat-i18n/i18n/ca.i18n.json

packages/rocketchat-i18n/i18n/cs.i18n.json

packages/rocketchat-i18n/i18n/cy.i18n.json

packages/rocketchat-i18n/i18n/da.i18n.json