DevTools: Remove unused CodeMirror line number border rule

This change fixes a bug where a misplaced blue bar appears in the middle
of the debugger when paused at a line.

The blue bar was intended to add color to the line gutter when paused on
that line. However, the "chevron" icon that appears on the line number
overlays the gutter border, so the rule has no visual effect even when
positioned correctly.

Thus, this change removes the CSS rule entirely as it is presently
positioned incorrectly, and has no effect when positioned correctly.

Before: https://i.imgur.com/cGluk3C.png
Positioned "correctly": https://i.imgur.com/xJzup61.png
After: https://i.imgur.com/RgpRIU1.png

Bug: 1005964
Change-Id: I701f6defca6a35dce2cedaccdc8ddf829598c4fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815853
Auto-Submit: Brian Cui <brcui@microsoft.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Mandy Chen <mandy.chen@microsoft.com>
Commit-Queue: Brian Cui <brcui@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#699389}

third_party/blink/renderer/devtools/front_end/text_editor/cmdevtools.css

@@ -309,15 +309,6 @@ div.CodeMirror:focus-within span.CodeMirror-nonmatchingbracket {
     background-color: rgb(171, 191, 254);
 }
 
-.cm-execution-line .CodeMirror-linenumber::after {
-    position: absolute;
-    top:-1px;
-    bottom:-1px;
-    right:-1.5em;
-    content:"";
-    border-right: 1px solid rgb(64, 115, 244);
-}
-
 :host-context(.-theme-with-dark-background) .cm-execution-line,
 .-theme-with-dark-background .cm-execution-line {
     background-color: #14522b;