resourceloader: Only output ResourceLoaderDynamicStyles when needed
In mediawiki.js, this marker has always been optional, falling back to
appending to <head>. When no stylesheets need to be after the marker
(e.g. no site styles on the wiki, and user is not logged-in), then
there is no need for the marker to exist.
In a previous refactor, I was going to do this and created an
"$append" variable in the function to do what this commit does,
but I forgot to actually use it for anything.
Test Plan:
* Local wiki, with no MediaWiki:Common/{Skinname}.css pages existing.
* When logged-out, before this change, there is a marker, now there is not.
* When creating "MediaWiki:Group-user.css" and logging in, there is still
a marker, and it is still above the <link> for that user styles request
in the <head>.
Bug: T219342
Change-Id: I2e9657f318088860916823efeb96ae4f1532974c