Revert of Convert enable_pre_sync_backup flag to new flags system.

components/sync_driver.gypi

Index: components/sync_driver.gypi
diff --git a/components/sync_driver.gypi b/components/sync_driver.gypi
index fa4aeb33da83950598b5557a054acbc8d992030d..7bda955703b62de168bf81779e2222148c0236cf 100644
--- a/components/sync_driver.gypi
+++ b/components/sync_driver.gypi
@@ -18,7 +18,6 @@
         'invalidation_public',
         'os_crypt',
         'signin_core_browser',
-        'sync_driver_features',
         'version_info',
       ],
       'include_dirs': [
@@ -153,26 +152,6 @@
       ],
     },
     {
-      # GN version: //components/sync_driver:features
-      'target_name': 'sync_driver_features',
-      'includes': [ '../build/buildflag_header.gypi' ],
-      'variables': {
-        'buildflag_header_path': 'components/sync_driver/sync_driver_features.h',
-        'variables': {
-          'conditions': [
-            ['OS=="win" or OS=="mac" or (OS=="linux" and chromeos==0)', {
-              'enable_pre_sync_backup%': 1,
-            }, {
-              'enable_pre_sync_backup%': 0,
-            }],
-          ],
-        },
-        'buildflag_flags': [
-          "ENABLE_PRE_SYNC_BACKUP=<(enable_pre_sync_backup)",
-        ],
-      },
-    },
-    {
       'target_name': 'sync_driver_test_support',
       'type': 'static_library',
       'dependencies': [