Remove views::SingleSplitView as it's presently unused.

ui/views/views.gyp

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 {
   'variables': {
     'chromium_code': 1,
     # Sources lists shared with GN build.
     'views_sources': [
       'accessibility/native_view_accessibility.cc',
       'accessibility/native_view_accessibility.h',
@@ skipping 180 matching lines @@
       'controls/scrollbar/native_scroll_bar.h',
       'controls/scrollbar/native_scroll_bar_views.cc',
       'controls/scrollbar/native_scroll_bar_views.h',
       'controls/scrollbar/native_scroll_bar_wrapper.h',
       'controls/scrollbar/overlay_scroll_bar.cc',
       'controls/scrollbar/overlay_scroll_bar.h',
       'controls/scrollbar/scroll_bar.cc',
       'controls/scrollbar/scroll_bar.h',
       'controls/separator.cc',
       'controls/separator.h',
-      'controls/single_split_view.cc',
-      'controls/single_split_view.h',
-      'controls/single_split_view_listener.h',
       'controls/slide_out_view.cc',
       'controls/slide_out_view.h',
       'controls/slider.cc',
       'controls/slider.h',
       'controls/styled_label.cc',
       'controls/styled_label.h',
       'controls/styled_label_listener.h',
       'controls/tabbed_pane/tabbed_pane.cc',
       'controls/tabbed_pane/tabbed_pane.h',
       'controls/tabbed_pane/tabbed_pane_listener.h',
@@ skipping 391 matching lines @@
       'controls/menu/menu_runner_cocoa_unittest.mm',
       'controls/menu/menu_runner_unittest.cc',
       'controls/native/native_view_host_mac_unittest.mm',
       'controls/native/native_view_host_test_base.cc',
       'controls/native/native_view_host_test_base.h',
       'controls/native/native_view_host_unittest.cc',
       'controls/prefix_selector_unittest.cc',
       'controls/progress_bar_unittest.cc',
       'controls/scroll_view_unittest.cc',
       'controls/scrollbar/scrollbar_unittest.cc',
-      'controls/single_split_view_unittest.cc',
       'controls/slider_unittest.cc',
       'controls/styled_label_unittest.cc',
       'controls/tabbed_pane/tabbed_pane_unittest.cc',
       'controls/table/table_utils_unittest.cc',
       'controls/table/table_view_unittest.cc',
       'controls/table/test_table_model.cc',
       'controls/table/test_table_model.h',
       'controls/textfield/textfield_model_unittest.cc',
       'controls/textfield/textfield_unittest.cc',
       'controls/tree/tree_view_unittest.cc',
@@ skipping 422 matching lines @@
                 '../aura/aura.gyp:aura',
                 '../wm/wm.gyp:wm',
               ],
             }],
           ],
         },  # target_name: macviews_interactive_ui_tests
       ],  # targets
     }],
   ],  # conditions
 }