UI: Revert default brush icon to monochrome version

Earlier in the brush assets project, for mesh sculpting, grease pencil,
and curves sculpting, the toolbar brush icon used was changed to a
version with a green highlight. This commit reverts that change to use
the monochrome grey icon instead in each of these modes.

For Sculpt mode specifically, the Paint tool icon is kept as this green
version to differentiate the usages. In future releases, further changes
are planned for sculpting related brushes to better represent their
functionality.

---

### Sculpt Mode Toolbar

![image](/attachments/f1575814-88f0-44f7-8ab6-7b069ff09763)

Pull Request: https://projects.blender.org/blender/blender/pulls/129934

scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -3079,6 +3079,15 @@ class IMAGE_PT_tools_active(ToolSelectPanelHelper, Panel):
     def tools_all(cls):
         yield from cls._tools.items()
 
+    _brush_tool = ToolDef.from_dict(
+        dict(
+            idname="builtin.brush",
+            label="Brush",
+            icon="brush.generic",
+            options={'USE_BRUSHES'},
+        )
+    )
+
     # Private tool lists for convenient reuse in `_tools`.
 
     _tools_transform = (
@@ -3135,7 +3144,7 @@ class IMAGE_PT_tools_active(ToolSelectPanelHelper, Panel):
             None,
         ],
         'PAINT': [
-            _defs_texture_paint.brush,
+            _brush_tool,
             _defs_texture_paint.blur,
             _defs_texture_paint.smear,
             _defs_texture_paint.fill,
@@ -3243,7 +3252,7 @@ class VIEW3D_PT_tools_active(ToolSelectPanelHelper, Panel):
         dict(
             idname="builtin.brush",
             label="Brush",
-            icon="brush.sculpt.paint",
+            icon="brush.generic",
             options={'USE_BRUSHES'},
         )
     )
@@ -3533,7 +3542,7 @@ class VIEW3D_PT_tools_active(ToolSelectPanelHelper, Panel):
             ),
         ],
         'PAINT_TEXTURE': [
-            _defs_texture_paint.brush,
+            _brush_tool,
             _defs_texture_paint.blur,
             _defs_texture_paint.smear,
             _defs_texture_paint.clone,

source/blender/editors/datafiles/CMakeLists.txt

@@ -24,6 +24,7 @@ set(LIB
 # See: `make icons_geom` and the script `./release/datafiles/blender_icons_geom_update.py`.
 set_property(GLOBAL PROPERTY ICON_GEOM_NAMES
   # BEGIN ICON_GEOM_NAMES
+  brush.generic
   brush.gpencil_draw.erase
   brush.gpencil_draw.fill
   brush.paint_texture.clone