Remove $body-text-shadow SCSS variable

Remove $body-text-shadow SCSS variable from _variables.scss and replace usage with appropriate GitLab UI design token or variable.

question: do we need it? https://gitlab.com/gitlab-org/gitlab/-/blob/a65de758f476bbcc83141be8eab70d90b41fab59/app/assets/stylesheets/framework/typography.scss#L697-699

body {
  -webkit-text-shadow: $body-text-shadow 0 0 1px;
}