fix(json-code-editor): remove opaque line-number layer background hiding editor content

The line-number layer spans the full editor, so giving it an opaque
background and raised z-index covered the highlighted content. Reverts
the gutter background added in 88868002f; the horizontal-scroll
gutter overlap it targeted returns as a known cosmetic issue.

web/src/styles/index.css

@@ -64,11 +64,6 @@ For commercial licensing, please contact support@quantumnous.com
     white-space: pre !important;
     white-space: pre !important;
   }
   }
 
 
-  .json-code-editor-lines {
-    background: var(--background);
-    z-index: 1;
-  }
-
   .json-code-editor-highlight .yace-tok--str {
   .json-code-editor-highlight .yace-tok--str {
     color: var(--chart-5);
     color: var(--chart-5);
   }
   }