[directorytree] Fix invalid file_types.css removable CSS rules

CL:1449472 added some invalid rules for removable volumes||partitions
see for example @line 474 which begins ...

   tree .tree-item > .tree-item > .tree-row >
   [volume-type-icon='removable'] {
     ...
   }

But .tree-item > .tree-item matches nothing in FilesApp: a .tree-item
is never a direct child of its parent .tree-item. So remove this rule
and now try to understand @line 466 of CL:1449472

   tree .tree-item[selected] > .tree-row >
   .tree-row[selected] [volume-type-icon='removable'][volume-sub...

Which might be a transcription error, and then the rule runs onto the
next line .tree-row > .tree-row[selected], but once again .tree-row >
.tree-row never matches anything in FilesApp. Remove and replace this
rule with an approximation of CL:1449472 intent.

Bug: 992819
Change-Id: I552890e021c02d1dd304e51a49b772eaf7f13546
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921595Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716089}

ui/file_manager/file_manager/foreground/css/file_types.css

@@ -480,7 +480,6 @@
       url(../images/volumes/2x/hard_drive.png) 2x);
 }
 
-tree .tree-item[selected] > .tree-row >
 .tree-row[selected] [volume-type-icon='removable'][volume-subtype='unknown'],
 .tree-item .tree-item[selected] [volume-type-icon='removable'] {
   background-image: -webkit-image-set(
@@ -488,13 +487,6 @@ tree .tree-item[selected] > .tree-row >
       url(../images/volumes/2x/hard_drive_active.png) 2x);
 }
 
-tree .tree-item > .tree-item > .tree-row >
-[volume-type-icon='removable'] {
-  background-image: -webkit-image-set(
-      url(../images/volumes/hard_drive_active.png) 1x,
-      url(../images/volumes/2x/hard_drive_active.png) 2x);
-}
-
 .tree-row:not([selected]) [volume-type-icon='provided'] {
   /* Third-party icons are not provided with both an active and a non-active
   version. This filter defines the non-active version of a provided icon. */