component/theia: don't set editor.multiCursorModifier

This was added in 48bcc52a (software/theia: some "better" defaults,
2019-08-30) because the default keybinding to create another cursor is
alt-click, but on ChromeOS alt-click does a right click, but:
 - not everybody use ChromeOS
 - this breaks the ctrl+click shortcut to go to definition/follow link
 - it makes SlapOS' Theia behaves differently from default Theia or vscode
 - ChromeOS users can use chrome://flags/#use-search-click-for-right-click
  (I'm using this flag and this seems perfect)