UI: Remove Masking dropdown from tool settings

In recent versions, masking is exposed as a popover in the header
so having this as well is confusing and takes up space.

scripts/startup/bl_ui/space_view3d_toolbar.py

@@ -712,7 +712,6 @@ class VIEW3D_PT_slots_vertex_groups(Panel):
 
 class VIEW3D_PT_mask(View3DPanel, Panel):
     bl_category = "Tool"
-    bl_context = ".imagepaint"  # dot on purpose (access from topbar)
     bl_label = "Masking"
     bl_options = {'DEFAULT_CLOSED'}