Drop custom SCSS now provided by Docsy 0.7.2

[chalin]

• Contributes a fix and closes Find a better way to use tabpane shortcodes #3127

Preview: e.g., tabbed panes in https://deploy-preview-3560--opentelemetry.netlify.app/docs/collector/installation/#docker are still styled correctly.

$ (cd public && git diff -bw --ignore-blank-lines) | grep ^diff                        
diff --git a/scss/main.css b/scss/main.css
diff --git a/scss/main.css.map b/scss/main.css.map
diff --git a/site/index.html b/site/index.html