Chat: remove unused variables for Visual Studio (#5275)

This commit updates the Visual Studio theme for webviews:

- Removes custom scrollbar styles for `.model-selector-popover`
elements, aligning with the default Visual Studio experience.
- Defines missing color variables for input validation and active
options, enhancing consistency with the Visual Studio theme.
- Commented out variables that are currently not used.

These changes improve the overall visual fidelity and consistency of the
webview themes.


## Test plan

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https://docs-legacy.sourcegraph.com/dev/background-information/testing_principles.
-->

Green UI - simple UI change. 

## Changelog

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https://www.notion.so/sourcegraph/Writing-a-changelog-entry-dd997f411d524caabf0d8d38a24a878c
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vscode/webviews/themes/visual-studio.css

@@ -631,22 +631,3 @@ html[data-ide='VisualStudio'] blockquote {
     background: var(--vscode-textBlockQuote-background);
     border-color: var(--vscode-textBlockQuote-border);
 }
-
-
-html[data-ide='VisualStudio'] .model-selector-popover::-webkit-scrollbar {
-    width: 10px;
-    height: 8px;
-    background-color: transparent;
-}
-
-/* color-mix is not supported on VisualStudio webview */
-html[data-ide='VisualStudio'] .context-item-mention-node {
-    color: var(--vscode-inputOption-activeForeground);
-    background-color: var(--vscode-badge-background);
-    gap: 0.2rem;
-
-    .icon {
-        width: 1rem;
-        height: 1rem;
-    }
-}