resourceloader: Remove creation of dynamic-styles marker

resourceloader: Remove creation of dynamic-styles marker

Firstly, this code is never used because the marker is unconditionally
created by OutputPage. But, we may change that one day, and we
want to reduce dependency on server-side specifics so that the
loader can (in theory) work on any web page. So we want to keep
some kind of fallback for now.

It's a private getter used in two places:

1. addEmbeddedCSS (private): Call newStyleTag with nextNode=marker.
   The newStyleTag function already treats nextNode as optional,
   and falls back to doing the same thing getMarker() was doing,
   which is: append to document.head.

2. addLink (private, debug-mode only): Used the marker as insertion
   point for a new element. Basically the same as newStyleTag,
   except it was using the wrapper of `$().before()` instead
   of calling Node#insertBefore() directly. Made it optional,
   with as fallback appending to document.head. Same as newStyleTag.

Also removed an unused jQuery object in addEmbeddedCSS, from
passing result of newStyleTag() to $(), but not using it.

Bug: T192623
Change-Id: If04c801c073b4cf74bf111d02ce3dc133bb862d1