[REF] *: BS4, get rid of the removed 'square' mixin

addons/hr_attendance/static/src/scss/hr_attendance.scss

addons/hr_org_chart/static/src/scss/hr_org_chart.scss

addons/mail/static/src/scss/discuss.scss

addons/mail/static/src/scss/mail_activity.scss

addons/web/static/src/scss/base_settings.scss

addons/website/static/src/scss/website.ui.scss

..._theme_install/static/src/scss/website_theme_install.scss

odoo/addons/base/static/src/scss/onboarding.scss