Remove disableMarkdown setting

This was used by the old composer to control whether to interpret text as markdown prior to sending.

The new setting is `MessageComposerInput.isRichTextEnabled`.

src/components/structures/UserSettings.js

@@ -89,10 +89,6 @@ const SETTINGS_LABELS = [
         id: 'hideRedactions',
         label: 'Hide removed messages',
     },
-    {
-        id: 'disableMarkdown',
-        label: 'Disable markdown formatting',
-    },
     {
         id: 'enableSyntaxHighlightLanguageDetection',
         label: 'Enable automatic language detection for syntax highlighting',