Refactor Git panel styling & status colors for consistency (#26951)

Closes #26847

Release Notes:

- Updated Git panel background to use panel_background instead of
ElevationIndex::Surface.bg(cx) for consistency with other panels.
- Removed redundant GitStatusColors struct from status.rs and refactored
to use existing theme colors.
- Adjusted Color enum mappings in color.rs to reference
version_control_* colors instead of status() for better alignment with
the theme system.
- Cleaned up unused or redundant code.

crates/theme/src/styles/status.rs

@@ -86,15 +86,6 @@ pub struct DiagnosticColors {
     pub info: Hsla,
 }
 
-pub struct GitStatusColors {
-    pub created: Hsla,
-    pub deleted: Hsla,
-    pub modified: Hsla,
-    pub renamed: Hsla,
-    pub conflict: Hsla,
-    pub ignored: Hsla,
-}
-
 impl StatusColors {
     pub fn dark() -> Self {
         Self {
@@ -197,15 +188,4 @@ impl StatusColors {
             info: self.info,
         }
     }
-
-    pub fn git(&self) -> GitStatusColors {
-        GitStatusColors {
-            created: self.created,
-            deleted: self.deleted,
-            modified: self.modified,
-            renamed: self.renamed,
-            conflict: self.conflict,
-            ignored: self.ignored,
-        }
-    }
 }